Cisco IOS IP Routing: BGP Command Reference

BGP Commands

address-family ipv4 (BGP)

To enter address family or router scope address family configuration mode to configure a routing session using standard IP Version 4 (IPv4) address prefixes, use the address-family ipv4 command in router configuration or router scope configuration mode. To exit address family configuration mode and remove the IPv4 address family configuration from the running configuration, use the no form of this command.

Syntax Available Under Router Configuration Mode

address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name]

no address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name]

Syntax Available Under Router Scope Configuration Mode

address-family ipv4 [mdt | multicast | unicast]

no address-family ipv4 [mdt | multicast | unicast]

Syntax Description

Command Default

IPv4 address prefixes are not enabled.

Command Modes

Router configuration (config-router)
Router scope configuration (config-router-scope)

Command History

Usage Guidelines

The address-family ipv4 command replaces the match nlri and set nlri commands. The address-family ipv4 command places the router in address family configuration mode (prompt: config-router-af), from which you can configure routing sessions that use standard IPv4 address prefixes. To leave address family configuration mode and return to router configuration mode, type exit.

Note Routing information for address family IPv4 is advertised by default for each BGP routing session configured with the neighbor remote-as command unless you enter the no bgp default ipv4-unicast command before configuring the neighbor remote-as command.

The tunnel keyword is used to enable the tunnel subaddress family identifier (SAFI) under the IPv4 address family identifier. This SAFI is used to advertise the tunnel endpoints and the SAFI-specific attributes (which contain the tunnel type and tunnel capabilities). Redistribution of tunnel endpoints into the BGP IPv4 tunnel SAFI table occurs automatically when the tunnel address family is configured. However, peers need to be activated under the tunnel address family before the sessions can exchange tunnel information.

The mdt keyword is used to enable the MDT SAFI under the IPv4 address family identifier. This SAFI is used to advertise tunnel endpoints for inter-AS multicast VPN peering sessions.

If you specify address-family ipv4 multicast, you will then specify the network network-number [mask network-mask] command. The network command advertises (injects) the specified network number and mask into the multicast BGP database. This route must exist in the forwarding table installed by an IGP (that is, by eigrp, ospf, rip, igrp, static, or is-is), but not bgp.

In Cisco IOS Release 12.2(33)SRB and later releases, the ability to use address family configuration under the router scope configuration mode was introduced. The scope hierarchy can be defined for BGP routing sessions and is required to support Multi-Topology Routing (MTR). To enter the router scope configuration mode, use the scope command, which can apply globally or for a specific VRF. When using the scope for a specific VRF, only the unicast keyword is available.

Examples

The following example places the router in address family configuration mode for the IPv4 address family:

Router(config)# router bgp 50000

Router(config-router)# address-family ipv4

Router(config-router-af)#

Multicast Example

The following example places the router in address family configuration mode and specifies only multicast address prefixes for the IPv4 address family:

Router(config)# router bgp 50000

Router(config-router)# address-family ipv4 multicast

Router(config-router-af)#

Unicast Example

The following example places the router in address family configuration mode and specifies unicast address prefixes for the IPv4 address family:

Router(config)# router bgp 50000

Router(config-router)# address-family ipv4 unicast

Router(config-router-af)#

VRF Example

The following example places the router in address family configuration mode and specifies cisco as the name of the VRF instance to associate with subsequent IPv4 address family configuration mode commands:

Router(config)# router bgp 50000

Router(config-router)# address-family ipv4 vrf cisco

Router(config-router-af)#

Note Use this form of the command, which specifies a VRF, only to configure routing exchanges between provider edge (PE) and customer edge (CE) devices.

Tunnel Example

The following example places the router in tunnel address family configuration mode:

Router(config)# router bgp 100

Router(config-router)# address-family ipv4 tunnel

Router(config-router-af)#

MDT Example

The following example shows how to configure a router to support an IPv4 MDT address-family session:

Router(config)# router bgp 45000

Router(config-router)# address-family ipv4 mdt

Router(config-router-af)#

Router Scope Configuration Mode Example

The following example shows how to configure the IPv4 address family under router scope configuration mode. In this example, the scope hierarchy is enabled globally. The router enters router scope address family configuration mode, and only multicast address prefixes for the IPv4 address family are specified:

Router(config)# router bgp 50000

Router(config-router)# scope global

Router(config-router-scope)# address-family ipv4 multicast

Router(config-router-scope-af)#

Related Commands

address-family l2vpn

To enter address family configuration mode to configure a routing session using Layer 2 Virtual Private Network (L2VPN) endpoint provisioning address information, use the address-family l2vpn command in router configuration mode. To remove the L2VPN address family configuration from the running configuration, use the no form of this command.

address-family l2vpn [vpls]

no address-family l2vpn [vpls]

Syntax Description

Command Default

No L2VPN endpoint provisioning support is enabled.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

The address-family l2vpn command places the router in address family configuration mode (prompt: config-router-af), from which you can configure routing sessions that support L2VPN endpoint provisioning.

BGP support for the L2VPN address family introduces a BGP-based autodiscovery mechanism to distribute L2VPN endpoint provisioning information. BGP uses a separate L2VPN routing information base (RIB) to store endpoint provisioning information, which is updated each time any Layer 2 (L2) virtual forwarding instance (VFI) is configured. Prefix and path information is stored in the L2VPN database, allowing BGP to make best-path decisions. When BGP distributes the endpoint provisioning information in an update message to all its BGP neighbors, the endpoint information is used to set up a pseudowire mesh to support L2VPN-based services.

The BGP autodiscovery mechanism facilitates the setting up of L2VPN services, which are an integral part of the Cisco IOS Virtual Private LAN Service (VPLS) feature. VPLS enables flexibility in deploying services by connecting geographically dispersed sites as a large LAN over high-speed Ethernet in a robust and scalable IP MPLS network.

Note Routing information for address family IPv4 is advertised by default for each BGP routing session configured with the neighbor remote-as command unless you configure the no bgp default ipv4-unicast command before configuring the neighbor remote-as command.

Examples

In this example, two provider edge (PE) routers are configured with VPLS endpoint provisioning information that includes L2 VFI, VPN, and VPLS IDs. BGP neighbors are configured and activated under L2VPN address family to ensure that the VPLS endpoint provisioning information is saved to a separate L2VPN RIB and then distributed to other BGP peers in BGP update messages. When the endpoint information is received by the BGP peers, a pseudowire mesh is set up to support L2VPN-based services.

Router A

enable

configure terminal

l2 vfi customerA autodiscovery

 vpn id 100

 vpls-id 45000:100

 exit

l2 vfi customerB autodiscovery

 vpn id 200

 vpls-id 45000:200

 exit

router bgp 45000

 no bgp default ipv4-unicast

 bgp log-neighbor-changes

 neighbor 172.16.1.2 remote-as 45000

 neighbor 172.21.1.2 remote-as 45000        

 address-family l2vpn vpls

 neighbor 172.16.1.2 activate               

 neighbor 172.16.1.2 send-community extended

 neighbor 172.21.1.2 activate               

 neighbor 172.21.1.2 send-community extended

 end

Router B

enable

configure terminal

l2 vfi customerA autodiscovery

 vpn id 100

 vpls-id 45000:100

 exit

l2 vfi customerB autodiscovery

 vpn id 200

 vpls-id 45000:200

 exit

router bgp 45000

 no bgp default ipv4-unicast

 bgp log-neighbor-changes

 neighbor 172.16.1.1 remote-as 45000

 neighbor 172.22.1.1 remote-as 45000        

 address-family l2vpn vpls

 neighbor 172.16.1.1 activate               

 neighbor 172.16.1.1 send-community extended

 neighbor 172.22.1.1 activate               

 neighbor 172.22.1.1 send-community extended

 end

Related Commands

address-family nsap

To enter address family configuration mode to configure Connectionless Network Service (CLNS)-specific parameters for Border Gateway Protocol (BGP) routing sessions, use the address-family nsap command in router configuration mode. To exit address family configuration mode and remove the CLNS address family configuration from the running configuration, use the no form of this command.

address-family nsap [unicast]

no address-family nsap [unicast]

Syntax Description

Command Default

NSAP prefix support is not enabled.

Note Routing information for address family IPv4 is advertised by default for each BGP routing session configured with the neighbor remote-as command unless you configure the no bgp default ipv4-unicast command before configuring the neighbor remote-as command.

Command Modes

Router configuration

Command History

Usage Guidelines

The address-family nsap command enters address family configuration mode (prompt: config-router-af)#, from which you can configure routing sessions that use standard NSAP address prefixes; you must enter NSAP address family configuration mode to configure BGP for CLNS prefixes.

To leave address family configuration mode and return to router configuration mode without removing the existing configuration, enter the exit-address-family command.

Examples

The following example enters NSAP address family configuration mode under BGP:

Router(config)# router bgp 50000

Router(config-router)# address-family nsap

Router(config-router-af)#

Related Commands

address-family rtfilter unicast

To enter address family configuration mode and to enable Automated Route Target Filtering with a BGP peer, use the address-family rtfilter unicast command in router configuration mode. To remove ARTF, use the no form of the command.

address-family rtfilter unicast

no address-family rtfilter unicast

Syntax Description

This command has no arguments or keywords.

Command Default

No RT Constraint support is enabled for BGP.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

Use this command when you are configuring the BGP: RT Constrained Route Distribution feature.

The address-family rtfilter unicast command is configured on the provider edge (PE) and route reflector (RR). The command enables the PE to send RT constraint (RTC) network layer reachability information (NLRI) to a route reflector (RR). As soon as you configure a peer as a RR client, the default filter and default route are sent out also.

Examples

In the following example, the local PE is configured to send RTC NLRI to the neighboring RR at 10.2.2.2:

router bgp 65000 

 address-family rtfilter unicast

 neighbor 10.2.2.2 activate

 exit-address-family

In the following example, the local PE is configured with the RT Constraint default filter, which indicates that the PE wants all of the VPN routes (regardless of the RT values):

router bgp 65000 

 address-family rtfilter unicast

 neighbor 10.2.2.2 activate

 neighbor 10.2.2.2 default-originate

 exit-address-family

In the following example, the RR is configured with the RT Constraint default filter, which indicates that the RR is requesting the PE to advertise all of its routes to the RR:

router bgp 65000 

 address-family rtfilter unicast

 neighbor 10.1.1.1 activate

 neighbor 10.1.1.1 default-originate

 exit-address-family

Related Commands

address-family vpnv4

To enter address family configuration mode to configure a routing session using Virtual Private Network (VPN) Version 4 address prefixes, use the address-family vpnv4 command in router configuration mode. To exit address family configuration mode and remove the VPNv4 address family configuration from the running configuration, use the no form of this command.

address-family vpnv4 [unicast]

no address-family vpnv4 [unicast]

Syntax Description

Defaults

Unicast prefix support is enabled by default when this command is entered without any optional keywords.

Note Routing information for address family IPv4 is advertised by default for each BGP routing session configured with the neighbor remote-as command unless you configure the no bgp default ipv4-unicast command before configuring the neighbor remote-as command.

Command Modes

Router configuration

Command History

Usage Guidelines

The address-family vpnv4 command replaces the match nlri and set nlri commands.

The address-family vpnv4 command places the router in address family configuration mode (prompt: config-router-af)#, from which you can configure routing sessions that use VPN Version 4 address prefixes.

To leave address family configuration mode and return to router configuration mode without removing the existing configuration, enter the exit-address-family command.

Examples

The following example places the router in address family configuration mode for the VPN Version 4 address family:

Router(config)# router bgp 50000

Router(config-router)# address-family vpnv4

Router(config-router-af)#

The following example places the router in address family configuration mode for the unicast VPN Version 4 address family:

Router(config)# router bgp 50000

Router(config-router)# address-family vpnv4 unicast

Router(config-router-af)#

Related Commands

aggregate-address

To create an aggregate entry in a Border Gateway Protocol (BGP) database, use the aggregate-address command in address family or router configuration mode. To disable this function, use the no form of this command.

aggregate-address address mask [as-set] [as-confed-set] [summary-only] [suppress-map map-name] [advertise-map map-name] [attribute-map map-name]

no aggregate-address address mask [as-set] [as-confed-set] [summary-only] [suppress-map map-name] [advertise-map map-name] [attribute-map map-name]

Syntax Description

Command Default

The atomic aggregate attribute is set automatically when an aggregate route is created with this command unless the as-set keyword is specified.

Command Modes

Address family configuration (config-router-af)
Router configuration (config-router)

Command History

Usage Guidelines

You can implement aggregate routing in BGP and Multiprotocol BGP (mBGP) either by redistributing an aggregate route into BGP or mBGP, or by using the conditional aggregate routing feature.

Using the aggregate-address command with no keywords will create an aggregate entry in the BGP or mBGP routing table if any more-specific BGP or mBGP routes are available that fall within the specified range. (A longer prefix that matches the aggregate must exist in the Routing Information Base (RIB).) The aggregate route will be advertised as coming from your autonomous system and will have the atomic aggregate attribute set to show that information might be missing. (By default, the atomic aggregate attribute is set unless you specify the as-set keyword.)

Using the as-set keyword creates an aggregate entry using the same rules that the command follows without this keyword, but the path advertised for this route will be an AS_SET consisting of all elements contained in all paths that are being summarized. Do not use this form of the aggregate-address command when aggregating many paths, because this route must be continually withdrawn and updated as autonomous system path reachability information for the summarized routes changes.

Using the as-confed-set keyword creates an aggregate entry using the same rules that the command follows without this keyword. This keyword performs the same function as the as-set keyword, except that it generates autonomous confed set path information.

Using the summary-only keyword not only creates the aggregate route (for example, 192.*.*.*) but also suppresses advertisements of more-specific routes to all neighbors. If you want to suppress only advertisements to certain neighbors, you may use the neighbor distribute-list command, with caution. If a more-specific route leaks out, all BGP or mBGP routers will prefer that route over the less-specific aggregate you are generating (using longest-match routing).

Using the suppress-map keyword creates the aggregate route but suppresses advertisement of specified routes. You can use the match clauses of route maps to selectively suppress some more-specific routes of the aggregate and leave others unsuppressed. IP access lists and autonomous system path access lists match clauses are supported.

Using the advertise-map keyword selects specific routes that will be used to build different components of the aggregate route, such as AS_SET or community. This form of the aggregate-address command is useful when the components of an aggregate are in separate autonomous systems and you want to create an aggregate with AS_SET, and advertise it back to some of the same autonomous systems. You must remember to omit the specific autonomous system numbers from the AS_SET to prevent the aggregate from being dropped by the BGP loop detection mechanism at the receiving router. IP access lists and autonomous system path access lists match clauses are supported.

Using the attribute-map keyword allows attributes of the aggregate route to be changed. This form of the aggregate-address command is useful when one of the routes forming the AS_SET is configured with an attribute such as the community no-export attribute, which would prevent the aggregate route from being exported. An attribute map route map can be created to change the aggregate attributes.

Examples

AS-Set Example

In the following example, an aggregate BGP address is created in router configuration mode. The path advertised for this route will be an AS_SET consisting of all elements contained in all paths that are being summarized.

Router(config)# router bgp 50000 

Router(config-router)# aggregate-address 10.0.0.0 255.0.0.0 as-set 

Summary-Only Example

In the following example, an aggregate BGP address is created in address family configuration mode and applied to the multicast database under the IP Version 4 address family. Because the summary-only keyword is configured, more-specific routes are filtered from updates.

Router(config)# router bgp 50000 

Router(config-router)# address-family ipv4 multicast 

Router(config-router-af)# aggregate-address 10.0.0.0 255.0.0.0 summary-only 

Conditional Aggregation Example

In the following example, a route map called MAP-ONE is created to match on an AS-path access list. The path advertised for this route will be an AS_SET consisting of elements contained in paths that are matched in the route map.

Router(config)# ip as-path access-list 1 deny ^1234_ 

Router(config)# ip as-path access-list 1 permit .* 

Router(config)# ! 

Router(config)# route-map MAP-ONE 

Router(config-route-map)# match ip as-path 1 

Router(config-route-map)# exit 

Router(config)# router bgp 50000 

Router(config-router)# address-family ipv4 

Router(config-router-af)# aggregate-address 10.0.0.0 255.0.0.0 as-set advertise-map 
MAP-ONE 

Router(config-router-af)# end 

Related Commands

auto-summary (BGP)

To configure automatic summarization of subnet routes into network-level routes, use the auto-summary command in address family or router configuration mode. To disable automatic summarization and send subprefix routing information across classful network boundaries, use the no form of this command.

auto-summary

no auto-summary

Syntax Description

This command has no arguments or keywords.

Command Default

Automatic summarization is disabled by default (the software sends subprefix routing information across classful network boundaries).

Command Modes

Address family configuration
Router configuration

Command History

Usage Guidelines

BGP automatically summarizes routes to classful network boundaries when this command is enabled. Route summarization is used to reduce the amount of routing information in routing tables. Automatic summarization applies to connected, static, and redistributed routes.

Note The MPLS VPN Per VRF Label feature does not support auto-summary.

By default, automatic summarization is disabled and BGP accepts subnets redistributed from an Interior Gateway Protocol (IGP). To block subnets and create summary subprefixes to the classful network boundary when crossing classful network boundaries, use the auto-summary command.

To advertise and carry subnet routes in BGP when automatic summarization is enabled, use an explicit network command to advertise the subnet. The auto-summary command does not apply to routes injected into BGP via the network command or through iBGP or eBGP.

Why auto-summary for BGP Is Disabled By Default

When auto-summary is enabled, routes injected into BGP via redistribution are summarized on a classful boundary. Remember that a 32-bit IP address consists of a network address and a host address. The subnet mask determines the number of bits used for the network address and the number of bits used for the host address. The IP address classes have a natural or standard subnet mask, as shown in Table 1.

Reserved addresses include 128.0.0.0, 191.255.0.0, 192.0.0.0, and 223.255.255.0.

When using the standard subnet mask, Class A addresses have one octet for the network, Class B addresses have two octets for the network, and Class C addresses have three octets for the network.

Consider the Class B address 156.26.32.1 with a 24-bit subnet mask, for example. The 24-bit subnet mask selects three octets, 156.26.32, for the network. The last octet is the host address. If the network 156.26.32.1/24 is learned via an IGP and is then redistributed into BGP, if auto-summary were enabled, the network would be automatically summarized to the natural mask for a Class B network. The network that BGP would advertise is 156.26.0.0/16. BGP would be advertising that it can reach the entire Class B address space from 156.26.0.0 to 156.26.255.255. If the only network that can be reached via the BGP router is 156.26.32.0/24, BGP would be advertising 254 networks that cannot be reached via this router. This is why the auto-summary (BGP) command is disabled by default.

Examples

In the following example, automatic summarization is enabled for IPv4 address family prefixes:

Router(config)# router bgp 50000 

Router(config-router)# address-family ipv4 unicast 

Router(config-router-af)# auto-summary 

Router(config-router-af)# network 7.7.7.7 255.255.255.255

In the example, there are different subnets, such as 7.7.7.6 and 7.7.7.7 on Loopback interface 6 and Loopback interface 7, respectively. Both auto-summary and a network command are configured.

Router# show ip interface brief

Interface              IP-Address      OK? Method Status                Protocol

Ethernet0/0            100.0.1.7       YES NVRAM  up                    up      

Ethernet0/1            unassigned      YES NVRAM  administratively down down    

Ethernet0/2            unassigned      YES NVRAM  administratively down down    

Ethernet0/3            unassigned      YES NVRAM  administratively down down    

Ethernet1/0            108.7.9.7       YES NVRAM  up                    up      

Ethernet1/1            unassigned      YES NVRAM  administratively down down    

Ethernet1/2            unassigned      YES NVRAM  administratively down down    

Ethernet1/3            unassigned      YES NVRAM  administratively down down    

Loopback6              7.7.7.6         YES NVRAM  up                    up      

Loopback7              7.7.7.7         YES NVRAM  up                    up      

Note that in the output below, because of the auto-summary command, the BGP routing table displays the summarized route 7.0.0.0 instead of 7.7.7.6. The 7.7.7.7/32 network is displayed because it was configured with the network command, which is not affected by the auto-summary command.

Router# show ip bgp

BGP table version is 10, local router ID is 7.7.7.7

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale, m multipath, b backup-path, x best-external

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 6.6.6.6/32       100.0.1.6                0             0 6 i

*> 7.0.0.0          0.0.0.0                  0         32768 ?   <-- summarization

*> 7.7.7.7/32       0.0.0.0                  0         32768 i   <-- network command

r>i9.9.9.9/32       108.7.9.9                0    100      0 i

*> 100.0.0.0        0.0.0.0                  0         32768 ?

r> 100.0.1.0/24     100.0.1.6                0             0 6 ?

*> 108.0.0.0        0.0.0.0                  0         32768 ?

r>i108.7.9.0/24     108.7.9.9                0    100      0 ?

*>i200.0.1.0        108.7.9.9 

Related Commands

bgp additional-paths install

To enable BGP to calculate a backup path for a given address family and to install it into the Routing Information Base (RIB) and Cisco Express Forwarding, use the bgp additional-paths install command in address family configuration or router configuration mode. To remove the backup paths, use the no form of this command.

bgp additional-paths install

no bgp additional-paths install

Syntax Description

This command has no arguments or keywords.

Command Default

A backup path is not created.

Command Modes

Address family configuration (config-router-af)
Router configuration (config-router)

Command History

Usage Guidelines

You can issue the bgp additional-paths install command in different modes, each of which protects VRFs in its own way:

•VPNv4 address family configuration mode protects all VRFs.

•IPv4 address family configuration mode protects only IPv4 VRFs.

•IPv6 address family configuration mode protects only IPv6 VRFs.

•Router configuration mode protects VRFs in the global routing table.

Examples

The following example shows how to calculate a backup path and install it into the RIB and Cisco Express Forwarding:

Router(config-router-af)# bgp additional-paths install

Related Commands

bgp additional-paths select

To have the system calculate a second BGP bestpath, use the bgp additional-paths select command in address family configuration mode. To remove this mechanism for calculating a second bestpath, use the no form of the command.

bgp additional-paths select {best-external [backup] | backup}

no bgp additional-paths select

Syntax Description

Command Default

This command is disabled by default.

Command Modes

Address family configuration (config-router-af)

Command History

Usage Guidelines

The BGP Diverse Path feature can be enabled on a route reflector to calculate a bestpath and an additional path per address family.

Computation of a diverse path per address family is triggered by any of the following commands:

•bgp additional-paths install

•bgp additional-paths select

•maximum-paths ebgp

•maximum-paths ibgp

The bgp additional-paths install command will install the type of path that is specified in the bgp additional-paths select command Either the best-external keyword or the backup keyword is required; both keywords can be specified. If both keywords (best-external and backup) are specified, the system will install a backup path.

Examples

In the following example, the system computes a second best path from among those received from external neighbors:

router bgp 1

 neighbor 10.1.1.1 remote-as 1

 address-family ipv4 unicast

 neighbor 10.1.1.1 activate

 maximum-paths ibgp 4

 bgp bestpath igp-metric ignore

 bgp additional-paths select best-external

 bgp additional-paths install

 neighbor 10.1.1.1 advertise diverse-path backup

Related Commands

bgp advertise-best-external

To enable BGP to calculate an external route as the best backup path for a given address family and to install it into the Routing Information base (RIB) and Cisco Express Forwarding, and to advertise the best external path to its neighbors, use the bgp advertise-best-external command in address family or router configuration mode. To remove the external backup path, use the no form of this command.

bgp advertise-best-external

no bgp advertise-best-external

Syntax Description

This command has no arguments or keywords.

Command Default

An external backup path is not created.

Command Modes

Router configuration (config-router)
Address family configuration (config-router-af)

Command History

Usage Guidelines

When you configure the Best External feature with the bgp advertise-best-external command, you need not enable the Prefix Independent Convergence (PIC) feature with the bgp additional-paths install command. The Best External feature automatically installs a backup path. If you try to configure the PIC feature after configuring the Best External feature, you receive an error. This behavior applies to both BGP and MPLS.

When you configure the MPLS VPN: Best External feature with the bgp advertise-best-external command, it will override the functionality of the MPLS VPN—BGP Local Convergence feature. You need not remove the protection local-prefixes command from the configuration.

You can issue the bgp advertise-best-external command in different modes, each of which protects VRFs in its own way:

•VPNv4 address-family configuration mode protects all VRFs.

•IPv4 address-family configuration mode protects only IPv4 VRFs.

•IPv6 address family configuration mode protects only IPv6 VRFs.

•Router configuration mode protects VRFs in the global routing table.

Examples

The following example calculates an external backup path and installs it into the RIB and Cisco Express Forwarding:

Router(config-router-af)# bgp advertise-best-external

Related Commands

bgp aggregate-timer

To set the interval at which BGP routes will be aggregated or to disable timer-based route aggregation, use the bgp aggregate-timer command in address-family or router configuration mode. To restore the default value, use the no form of this command.

bgp aggregate-timer seconds

no bgp aggregate-timer

Syntax Description

Command Default

30 seconds

Command Modes

Address family configuration (config-router-af)
Router configuration (config-router)

Command History

Usage Guidelines

Use this command to change the default interval at which BGP routes are aggregated.

In very large configurations, even if the aggregate-address summary-only command is configured, more specific routes are advertised and later withdrawn. To avoid this behavior, configure the bgp aggregate-timer to 0 (zero), and the system will immediately check for aggregate routes and suppress specific routes.

Examples

The following example configures BGP route aggregation at 20-second intervals:

Router(config)# router bgp 50

Router(config-router)# bgp aggregate-timer 20

The following example starts BGP route aggregation immediately:

Router(config)# router bgp 50

Router(config-router)# aggregate-address 10.0.0.0 255.0.0.0 summary-only

Router(config-router)# bgp aggregate-timer 0

Related Commands

bgp always-compare-med

To enable the comparison of the Multi Exit Discriminator (MED) for paths from neighbors in different autonomous systems, use the bgp always-compare-med command in router configuration mode. To disallow the comparison, use the no form of this command.

bgp always-compare-med

no bgp always-compare-med

Syntax Description

This command has no arguments or keywords.

Command Default

Cisco IOS software does not compare the MED for paths from neighbors in different autonomous systems if this command is not enabled or if the no form of this command is entered. The MED is compared only if the autonomous system path for the compared routes is identical.

Command Modes

Router configuration

Command History

Usage Guidelines

The MED, as stated in RFC 1771, is an optional nontransitive attribute that is a four octet non-negative integer. The value of this attribute may be used by the BGP best path selection process to discriminate among multiple exit points to a neighboring autonomous system.

The MED is one of the parameters that is considered when selecting the best path among many alternative paths. The path with a lower MED is preferred over a path with a higher MED. During the best-path selection process, MED comparison is done only among paths from the same autonomous system. The bgp always-compare-med command is used to change this behavior by enforcing MED comparison between all paths, regardless of the autonomous system from which the paths are received.

The bgp deterministic-med command can be configured to enforce deterministic comparison of the MED value between all paths received from within the same autonomous system.

Examples

In the following example, the local BGP routing process is configured to compare the MED from alternative paths, regardless of the autonomous system from which the paths are received:

Router(config)# router bgp 500000 

Router(config-router)# bgp always-compare-med 

Related Commands

bgp asnotation dot

To change the default display and regular expression match format of Border Gateway Protocol (BGP) 4-byte autonomous system numbers from asplain (decimal values) to dot notation, use the bgp asnotation dot command in router configuration mode. To reset the default 4-byte autonomous system number display and regular expression match format to asplain, use the no form of this command.

bgp asnotation dot

no bgp asnotation dot

Syntax Description

This command has no arguments or keywords.

Command Default

BGP autonomous system numbers are displayed using asplain (decimal value) format in screen output, and the default format for matching 4-byte autonomous system numbers in regular expressions is asplain.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

Prior to January 2009, BGP autonomous system numbers that were allocated to companies were 2-octet numbers in the range from 1 to 65535 as described in RFC 4271, A Border Gateway Protocol 4 (BGP-4). Due to increased demand for autonomous system numbers, the Internet Assigned Number Authority (IANA) will start in January 2009 to allocate four-octet autonomous system numbers in the range from 65536 to 4294967295. RFC 5396, Textual Representation of Autonomous System (AS) Numbers, documents three methods of representing autonomous system numbers. Cisco has implemented the following two methods:

•Asplain—Decimal value notation where both 2-byte and 4-byte autonomous system numbers are represented by their decimal value. For example, 65526 is a 2-byte autonomous system number and 234567 is a 4-byte autonomous system number.

•Asdot—Autonomous system dot notation where 2-byte autonomous system numbers are represented by their decimal value and 4-byte autonomous system numbers are represented by a dot notation. For example, 65526 is a 2-byte autonomous system number and 1.169031 is a 4-byte autonomous system number (this is dot notation for the 234567 decimal number).

For details about the third method of representing autonomous system numbers, see RFC 5396.

In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, and later releases, the Cisco implementation of 4-byte autonomous system numbers uses asplain as the default display format for autonomous system numbers, but you can configure 4-byte autonomous system numbers in both the asplain and asdot format. In addition, the default format for matching 4-byte autonomous system numbers in regular expressions is asplain, so you must ensure that any regular expressions to match 4-byte autonomous system numbers are written in the asplain format. If you want to change the default show command output to display 4-byte autonomous system numbers in the asdot format, use the bgp asnotation dot command under router configuration mode. When the asdot format is enabled as the default, any regular expressions to match 4-byte autonomous system numbers must be written using the asdot format, or the regular expression match will fail. Table 2 and Table 3 show that although you can configure 4-byte autonomous system numbers in either asplain or asdot format, only one format is used to display show command output and control 4-byte autonomous system number matching for regular expressions, and the default is asplain format. To display 4-byte autonomous system numbers in show command output and to control matching for regular expressions in the asdot format, you must configure the bgp asnotation dot command. After enabling the bgp asnotation dot command, a hard reset must be initiated for all BGP sessions by entering the clear ip bgp * command.

Note If you are upgrading to an image that supports 4-byte autonomous system numbers, you can still use 2-byte autonomous system numbers. The show command output and regular expression match are not changed and remain in asplain (decimal value) format for 2-byte autonomous system numbers regardless of the format configured for 4-byte autonomous system numbers.

Examples

The following output from the show ip bgp summary command shows the default asplain format of the 4-byte autonomous system numbers. Note the asplain format of the 4-byte autonomous system numbers, 65536 and 65550.

Router# show ip bgp summary

BGP router identifier 172.17.1.99, local AS number 65538

BGP table version is 1, main routing table version 1

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  Statd

192.168.1.2     4       65536       7       7        1    0    0 00:03:04      0

192.168.3.2     4       65550       4       4        1    0    0 00:00:15      0

The following configuration is performed to change the default output format to the asdot notation format:

configure terminal

 router bgp 65538

  bgp asnotation dot

  end

clear ip bgp *

After the configuration is performed, the output is converted to asdot notation format as shown in the following output from the show ip bgp summary command. Note the asdot format of the 4-byte autonomous system numbers, 1.0 and 1.14 (these are the asdot conversions of the 65536 and 65550 autonomous system numbers).

Router# show ip bgp summary

BGP router identifier 172.17.1.99, local AS number 1.2

BGP table version is 1, main routing table version 1

Neighbor        V          AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  Statd

192.168.1.2     4         1.0       9       9        1    0    0 00:04:13     0

192.168.3.2     4        1.14       6       6        1    0    0 00:01:24     0

After the bgp asnotation dot command is configured, the regular expression match format for 4-byte autonomous system paths is changed to asdot notation format. Although a 4-byte autonomous system number can be configured in a regular expression using either asplain format or asdot format, only 4-byte autonomous system numbers configured using the current default format are matched. In the first example, the show ip bgp regexp command is configured with a 4-byte autonomous system number in asplain format. The match fails because the default format is currently asdot format and there is no output. In the second example using asdot format, the match passes and the information about the 4-byte autonomous system path is shown using the asdot notation.

Note The asdot notation uses a period, which is a special character in Cisco regular expressions. To remove the special meaning, use a backslash before the period.

Router# show ip bgp regexp ^65536$

Router# show ip bgp regexp ^1\.0$

BGP table version is 2, local router ID is 172.17.1.99

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 10.1.1.0/24      192.168.1.2              0             0 1.0 i

Related Commands

bgp bestpath as-path ignore

To configure Border Gateway Protocol (BGP) to not consider the autonomous system (AS) path during best path route selection, use the bgp bestpath as-path ignore command in router configuration mode. To restore default behavior and configure BGP to consider the AS-path during route selection, use the no form of this command.

bgp bestpath as-path ignore

no bgp bestpath as-path ignore

Syntax Description

This command has no arguments or keywords.

Command Default

The AS-path is considered during BGP best path selection.

Command Modes

Router configuration

Command History

Examples

In the following example, the BGP routing process is configured to not consider the AS-path during best path selection:

Router(config)# router bgp 40000

Router(config-router)# bgp bestpath as-path ignore

Related Commands

bgp bestpath compare-routerid

To configure a Border Gateway Protocol (BGP) routing process to compare identical routes received from different external peers during the best path selection process and to select the route with the lowest router ID as the best path, use the bgp bestpath compare-routerid command in router configuration mode. To return the BGP routing process to the default operation, use the no form of this command.

bgp bestpath compare-routerid

no bgp bestpath compare-routerid

The behavior of this command is disabled by default; BGP selects the route that was received first when two routes with identical attributes are received.

Command HistoryRouter configuration

Usage Guidelines

The bgp bestpath compare-routerid command is used to configure a BGP routing process to use the router ID as the tie breaker for best path selection when two identical routes are received from two different peers (all the attributes are the same except for the router ID). When this command is enabled, the lowest router ID will be selected as the best path when all other attributes are equal.

In the following example, the BGP routing process is configured to compare and use the router ID as a tie breaker for best path selection when identical paths are received from different peers:

Router(config)# router bgp 50000 

Router(config-router)# bgp bestpath compare-routerid 

Related Commands

bgp bestpath cost-community ignore

To configure a router that is running the Border Gateway Protocol (BGP) to not evaluate the cost community attribute during the best path selection process, use the bgp bestpath cost-community ignore command in router configuration mode. To return the router to default operation, use the no form of this command.

bgp bestpath cost-community ignore

no bgp bestpath cost-community ignore

Syntax Description

This command has no keywords or arguments.

Command Default

The behavior of this command is enabled by default until the cost community attribute is manually configured.

Command Modes

Address family configuration
Router configuration

Command History

Usage Guidelines

The bgp bestpath cost-community ignore command is used to disable the evaluation of the cost community attribute to help isolate problems and troubleshoot issues that relate to BGP path selection. This command can also be used to delay the activation of cost community attribute evaluation so that cost community filtering can be deployed in a large network at the same time.

Examples

The following example shows how to configure a router to not evaluate the cost community attribute during the best path selection process:

router bgp 50000

 address-family ipv4 unicast 

 bgp bestpath cost-community ignore

Related Commands

bgp bestpath igp-metric ignore

To have the system ignore the Interior Gateway Protocol (IGP) metric during BGP best path selection, use the bgp bestpath igp-metric ignore command in address family configuration mode. To remove the instruction to ignore the IGP metric, use the no form of this command.

bgp bestpath igp-metric ignore

no bgp bestpath igp-metric ignore

Syntax Description

This command has no arguments or keywords.

Command Default

This command is disabled by default.

Command Modes

Address family configuration (config-router-af)

Command History

Usage Guidelines

The IGP metric is a configurable metric for EIGRP, IS-IS, or OSPF that is related to distance. The bgp bestpath igp-metric ignore command can be used independently, or in conjunction with the BGP Diverse Path feature. This command does not enable the BGP Diverse Path feature.

Similarly, enabling the BGP Diverse Path feature does not necessarily require that the IGP metric be ignored. If you enable the BGP Diverse Path feature and the RR and its shadow RR are not co-located, this command must be configured on the RR, shadow RR, and PE routers.

This command is supported in the following address families:

•ipv4 unicast

•vpnv4 unicast

•ipv6 unicast

•vpnv6 unicast

•ipv4+label

•ipv6+label

Note This command is not supported per VRF; if you use it per VRF, it is at your own risk.

This command applies per VRF as follows (which is consistent with the BGP PIC/Best External feature):

•When configured under address-family vpnv4 or vpnv6, it applies to all VRFs, but it will be nvgened only under vpnv4/vpnv6 global.

•When configured under a particular VRF, it applies only to that VRF and will be nvgened only for that VRF.

•When configured under vpnv4 or vpnv6 global, this command can be disabled for a particular VRF by specifying no bgp bestpath igp-metric ignore. The no form will be nvgened under that VRF, while under vpnv4 or vpnv6 bgp bestpath igp-metric ignore is nvgened and the command applies to all other VRFs.

Examples

In the following example, the IGP metric is ignored during calculation of the BGP best path:

router bgp 1

 neighbor 10.1.1.1 remote-as 1

 address-family ipv4 unicast

 neighbor 10.1.1.1 activate

 maximum-paths ibgp 4

 bgp bestpath igp-metric ignore

 bgp additional-paths select backup

 bgp additional-paths install

 neighbor 10.1.1.1 advertise diverse-path backup

Related Commands

bgp bestpath med confed

To configure a Border Gateway Protocol (BGP) routing process to compare the Multi Exit Discriminator (MED) between paths learned from confederation peers, use the bgp bestpath med confed command in router configuration mode. To disable MED comparison of paths received from confederation peers, use the no form of this command.

bgp bestpath med confed [missing-as-worst]

no bgp bestpath med confed [missing-as-worst]

Syntax Description

Defaults

Cisco IOS software does not consider the MED attribute when choosing among paths learned from confederation peers if this command is not enabled or if the no form of this command is entered.

Command Modes

Router configuration

Command History

Usage Guidelines

The MED comparison between confederation peers occurs only if no external autonomous systems are in the path (an external autonomous system is an autonomous system that is not within the confederation). If an external autonomous system in the path, then the external MED is passed transparently through the confederation, and the comparison is does not occur.

For example, assume that autonomous system 65000, 65001, 65002, and 65004 are part of the confederation; autonomous system 1 is not; and we are comparing route A with four paths. If the bgp bestpath med confed command is enabled, path 1 would be chosen. The fourth path has a lower MED, but it is not involved in the MED comparison because there is an external autonomous system in this path. The following list displays the MED for each autonomous system.

path = 65000 65004, med = 2

path = 65001 65004, med = 3

path = 65002 65004, med = 4

path = 65003 1, med = 1

Examples

In the following example, the BGP routing process is configured to compare MED values for paths learned from confederation peers:

Router(config)# router bgp 50000 

Router(config-router)# bgp bestpath med confed 

Related Commands

bgp bestpath med missing-as-worst

To configure a Border Gateway Protocol (BGP) routing process to assign a value of infinity to routes that are missing the Multi Exit Discriminator (MED) attribute (making the path without a MED value the least desirable path), use the bgp bestpath med missing-as-worst command in router configuration mode. To return the router to the default behavior (assign a value of 0 to the missing MED), use the no form of this command.

bgp bestpath med missing-as-worst

no bgp bestpath med missing-as-worst

Syntax Description

This command has no arguments or keywords.

Defaults

Cisco IOS software assigns a value of 0 to routes the are missing the MED attribute, causing the route with the missing MED attribute to be considered the best path.

Command Modes

Router configuration

Command History

Examples

In the following example, the BGP router process is configured to consider a route with a missing MED attribute as having a value of infinity (4294967294), making this path the least desirable path:

Router(config)# router bgp 50000 

Router(config-router)# bgp bestpath med missing-as-worst 

Related Commands

bgp client-to-client reflection

To enable or restore route reflection from a BGP route reflector to clients, use the bgp client-to-client reflection command in router configuration mode. To disable client-to-client route reflection, use the no form of this command.

bgp client-to-client reflection

no bgp client-to-client reflection

Syntax Description

This command has no arguments or keywords.

Defaults

Client-to-client route reflection is enabled by default; when a route reflector is configured, the route reflector reflects routes from a client to other clients.

Command Modes

Router configuration

Command History

Usage Guidelines

By default, the clients of a route reflector are not required to be fully meshed and the routes from a client are reflected to other clients. However, if the clients are fully meshed, route reflection is not required. In this case, use the no bgp client-to-client reflection command to disable client-to-client reflection.

Examples

In the following example, the local router is a route reflector, and the three neighbors are fully meshed. Because the neighbors are fully meshed, client-to-client reflection is disabled with the no bgp client-to-client reflection command.

Router(config)# router bgp 50000

Router(config-router)# neighbor 10.24.95.22 route-reflector-client

Router(config-router)# neighbor 10.24.95.23 route-reflector-client

Router(config-router)# neighbor 10.24.95.24 route-reflector-client

Router(config-router)# no bgp client-to-client reflection

Router(config-router)# end

Related Commands

bgp cluster-id

To set the cluster ID on a route reflector in a route reflector cluster, use the bgp cluster-id command in router configuration mode. To remove the cluster ID, use the no form of this command.

bgp cluster-id cluster-id

no bgp cluster-id cluster-id

Syntax Description

Defaults

The local router ID of the route reflector is used as the cluster ID when no ID is specified or when the no form of this command is entered.

Command Modes

Router configuration

Command History

Usage Guidelines

Together, a route reflector and its clients form a cluster. When a single route reflector is deployed in a cluster, the cluster is identified by the router ID of the route reflector.

The bgp cluster-id command is used to assign a cluster ID to a route reflector when the cluster has one or more route reflectors. Multiple route reflectors are deployed in a cluster to increase redundancy and avoid a single point of failure. When multiple route reflectors are configured in a cluster, the same cluster ID is assigned to all route reflectors. This allows all route reflectors in the cluster to recognize updates from peers in the same cluster and reduces the number of updates that need to be stored in BGP routing tables.

Note All route reflectors must maintain stable sessions between all peers in the cluster. If stable sessions cannot be maintained, then overlay route reflector clusters should be used instead (route reflectors with different cluster IDs).

Examples

In the following example, the local router is one of the route reflectors serving the cluster. It is configured with the cluster ID to identify the cluster.

Router(config)# router bgp 50000 

Router(config-router)# neighbor 192.168.70.24 route-reflector-client 

Router(config-router)# bgp cluster-id 10.0.1.2 

Related Commands

bgp confederation identifier

To specify a BGP confederation identifier, use the bgp confederation identifier command in router configuration mode. To remove the confederation identifier, use the no form of this command.

bgp confederation identifier autonomous-system-number

no bgp confederation identifier autonomous-system-number

Syntax Description

Command Default

No BGP confederation identifier is identified.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

The bgp confederation identifier command is used to configure a single autonomous system number to identify a group of smaller autonomous systems as a single confederation.

A confederation can be used to reduce the internal BGP (iBGP) mesh by dividing a large single autonomous system into multiple subautonomous systems and then grouping them into a single confederation. The subautonomous systems within the confederation exchange routing information like iBGP peers. External peers interact with the confederation as if it were a single autonomous system.

Each subautonomous system is fully meshed within itself and has a few connections to other autonomous systems within the confederation. Next hop, Multi Exit Discriminator (MED), and local preference information is preserved throughout the confederation, allowing you to retain a single Interior Gateway Protocol (IGP) for all the autonomous systems.

In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, and later releases, the Cisco implementation of 4-byte autonomous system numbers uses asplain—65538 for example—as the default regular expression match and output display format for autonomous system numbers, but you can configure 4-byte autonomous system numbers in both the asplain format and the asdot format as described in RFC 5396. To change the default regular expression match and output display of 4-byte autonomous system numbers to asdot format, use the bgp asnotation dot command followed by the clear ip bgp * command to perform a hard reset of all current BGP sessions.

In Cisco IOS Release 12.0(32)S12, 12.4(24)T, and Cisco IOS XE Release 2.3, the Cisco implementation of 4-byte autonomous system numbers uses asdot—1.2 for example—as the only configuration format, regular expression match, and output display, with no asplain support.

If one member of a BGP confederation is identified using a 4-byte autonomous system number, all other members of a BGP confederation must be upgraded to support 4-byte autonomous system numbers.

Examples

In the following example, the routing domain is divided into autonomous systems 50001, 50002, 50003, 50004, 50005, and 50006 and is identified by the confederation identifier 50007. Neighbor 10.2.3.4 is a peer inside of the routing domain confederation. Neighbor 10.4.5.6 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 50007.

router bgp 50000 

 bgp confederation identifier 50007 

 bgp confederation peers 50001 50002 50003 50004 50005 50006 

 neighbor 10.2.3.4 remote-as 50001 

 neighbor 10.4.5.6 remote-as 40000 

 end 

In the following example, the routing domain is divided into autonomous systems using 4-byte autonomous system numbers 65538, 65536, and 65550 in asplain format and identified by the confederation identifier 65545. Neighbor 192.168.1.2 is a peer inside of the routing domain confederation. Neighbor 192.168.2.2 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 65545. This example requires Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, or a later release.

router bgp 65550 

 bgp confederation identifier 65545

 bgp confederation peers 65538 65536 65550

 neighbor 192.168.1.2 remote-as 65536

 neighbor 192.168.2.2 remote-as 65547

 end 

In the following example, the routing domain is divided into autonomous systems using 4-byte autonomous system numbers 1.2 and 1.0 in asdot format and is identified by the confederation identifier 1.9. Neighbor 192.168.1.2 is a peer inside of the routing domain confederation. Neighbor 192.168.2.2 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 1.9. This example requires Cisco IOS Release 12.0(32)S12, 12.4(24)T, or Cisco IOS XE Release 2.3 where asdot notation is the only format for 4-byte autonomous system numbers. This configuration can also be performed using Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, or later releases.

router bgp 1.14

 bgp confederation identifier 1.9

 bgp confederation peers 1.2 1.0

 neighbor 192.168.1.2 remote-as 1.0

 neighbor 192.168.2.2 remote-as 1.11

 end 

Related Commands

bgp confederation peers

To configure subautonomous systems to belong to a single confederation, use the bgp confederation peers command in router configuration mode. To remove an autonomous system from the confederation, use the no form of this command.

bgp confederation peers autonomous-system-number [... autonomous-system-number]

no bgp confederation peers autonomous-system-number [... autonomous-system-number]

Syntax Description

Command Default

No BGP peers are configured to be members of a BGP confederation.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

The bgp confederation peers command is used to configure multiple autonomous systems as a single confederation. The ellipsis (...) in the command syntax indicates that your command input can include multiple values for the autonomous-system-number argument.

The autonomous system number of the router on which this command is being specified is not allowed in this command (not allowed as a confederation peer). If you specify the local router's autonomous system number in the bgp confederation peers command, the error message "Local member-AS not allowed in confed peer list" will appear.

The autonomous systems specified in this command are visible internally to the confederation. Each autonomous system is fully meshed within itself. Use the bgp confederation identifier command to specify the confederation to which the autonomous systems belong.

In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4 , and later releases, the Cisco implementation of 4-byte autonomous system numbers uses asplain—65538 for example—as the default regular expression match and output display format for autonomous system numbers, but you can configure 4-byte autonomous system numbers in both the asplain format and the asdot format as described in RFC 5396. To change the default regular expression match and output display of 4-byte autonomous system numbers to asdot format, use the bgp asnotation dot command followed by the clear ip bgp * command to perform a hard reset of all current BGP sessions.

In Cisco IOS Release 12.0(32)S12, 12.4(24)T, and Cisco IOS XE Release 2.3, the Cisco implementation of 4-byte autonomous system numbers uses asdot—1.2 for example—as the only configuration format, regular expression match, and output display, with no asplain support.

If one member of a BGP confederation is identified using a 4-byte autonomous system number, all other members of a BGP confederation must be upgraded to support 4-byte autonomous system numbers.

Examples

In the following example, autonomous systems 50001, 50002, 50003, 50004, and 50005 are configured to belong to a single confederation under the identifier 50000:

router bgp 50000 

 bgp confederation identifier 50000 

 bgp confederation peers 50001 50002 50003 50004 50005 

In the following example, the routing domain is divided into autonomous systems using 4-byte autonomous system numbers 65538 and 65536, and is identified by the confederation identifier 65545. Neighbor 192.168.1.2 is a peer inside of the routing domain confederation. Neighbor 192.168.2.2 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 65545. This example requires Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, or a later release.

router bgp 65550 

 bgp confederation identifier 65545

 bgp confederation peers 65538 65536

 neighbor 192.168.1.2 remote-as 65536

 neighbor 192.168.2.2 remote-as 65547

 end 

In the following example, the routing domain is divided into autonomous systems using 4-byte autonomous system numbers 1.2, 1.0, and 1.14 and is identified by the confederation identifier 1.9. Neighbor 192.168.1.2 is a peer inside of the routing domain confederation. Neighbor 192.168.2.2 is a peer outside of the routing domain confederation. To external peers and routing domains, the confederation appears as a single autonomous system with the number 1.9. This example requires Cisco IOS Release 12.0(32)S12, 12.4(24)T, or Cisco IOS XE Release 2.3 where asdot notation is the only format for 4-byte autonomous system numbers. This configuration can also be performed using Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, Cisco IOS XE Release 2.4, or later releases.

router bgp 1.14 

 bgp confederation identifier 1.9

 bgp confederation peers 1.2 1.0 1.14

 neighbor 192.168.1.2 remote-as 1.0

 neighbor 192.168.2.2 remote-as 1.11

 end 

Related Commands

bgp consistency-checker

To enable the BGP Consistency Checker feature, use the bgp consistency-checker command in router configuration mode. To disable the BGP Consistency Checker feature, use the no form of this command.

bgp consistency-checker {error-message | auto-repair} [interval minutes]

no bgp consistency-checker

Syntax Description

Command Default

No BGP consistency check is performed.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

A BGP route inconsistency with a peer occurs when an update or a withdraw is not sent to a peer, and black-hole routing can result. The BGP consistency checker feature is a low-priority process created to address this issue. This feature performs nexthop-label, RIB-out, and aggregation consistency checks. When BGP consistency checker is enabled, it is performed for all address families. Once the process identifies such an inconsistency:

•If the error-message keyword is specified, the system will report the inconsistency with a syslog message, and will also perform forceful aggregation reevaluation in the case of an aggregation inconsistency.

•If the auto-repair keyword is specified, the system will report the inconsistency with a syslog message and also take appropriate action, such as a route refresh request or an aggregation reevaluation, depending on the type of inconsistency.

Examples

In the following example, BGP consistency checker is enabled. If a BGP route inconsistency is found, the system will send a syslog message and take appropriate action.

Router(config)# router bgp 65000

Router(config-router)# bgp consistency-checker auto-repair

Related Commands

bgp dampening

To enable BGP route dampening or change BGP route dampening parameters, use the bgp dampening command in address family or router configuration mode. To disable BGP dampening, use the no form of this command.

bgp dampening [half-life reuse suppress max-suppress-time | route-map map-name]

no bgp dampening [half-life reuse suppress max-suppress-time | route-map map-name]

Syntax Description

Defaults

BGP dampening is disabled by default. The following values are used when this command is enabled without configuring any optional arguments:

half-life: 15 minutes
reuse: 750
suppress: 2000
max-suppress-time: 4 times half-life

Command Modes

Address family configuration
Router configuration

Command History

Usage Guidelines

The bgp dampening command is used to enable BGP route dampening. This command can be entered without any arguments or keywords. The half-life, reuse, suppress, and max-suppress-time arguments are position-dependent; meaning that if any of these arguments are entered, then all optional arguments must be entered.

When BGP dampening is configured and a prefix is withdrawn, BGP considers the withdrawn prefix as a flap and increases the penalty by a 1000. If BGP receives an attribute change, BGP increases the penalty by 500. If then the prefix has been withdrawn, BGP keeps the prefix in the BGP table as a history entry. If the prefix has not been withdrawn by the neighbor and BGP is not using this prefix, the prefix is marked as dampened. Dampened prefixes are not used in the BGP decision process and not installed to the routing table.

Note This command is not supported in the address family configuration mode in Cisco IOS Release 12.2SX and later releases.

Examples

In the following example, the BGP dampening values are set to 30 minutes for the half life, 1500 for the reuse value, 10000 for the suppress value, and 120 minutes for the maximum suppress time:

Router(config)# router bgp 5

Router(config-router)# address-family ipv4 unicast 

Router(config-router-af)# bgp dampening 30 1500 10000 120

Router(config-router-af)# end 

In the following example, BGP dampening is applied to prefixes filtered through the route-map named BLUE:

Router(config)# ip prefix-list RED permit 10.0.0.0/8 

Router(config)# !

Router(config)# route-map BLUE 

Router(config-route-map)# match ip address ip prefix-list RED 

Router(config-route-map)# exit

Router(config)# router bgp 50000 

Router(config-router)# address-family ipv4 

Router(config-router-af)# bgp dampening route-map BLUE 

Router(config-router-af)# end 

Related Commands

bgp default ipv4-unicast

To set the IP version 4 (IPv4) unicast address family as default for BGP peering session establishment, use the bgp default ipv4-unicast command in router configuration mode. To disable default IPv4 unicast address family for peering session establishment, use the no form of this command.

bgp default ipv4-unicast

no bgp default ipv4-unicast

Syntax Description

This command has no arguments or keywords.

Command Default

IPv4 address family routing information is advertised by default for each BGP routing session configured with the neighbor remote-as command, unless you first configure the no bgp default ipv4-unicast command before configuring the neighbor remote-as command.

Command Modes

Router configuration

Command History

Usage Guidelines

The bgp default ipv4-unicast command is used to enable the automatic exchange of IPv4 address family prefixes. The neighbor activate address family configuration command must be entered in each IPv4 address family session before prefix exchange will occur.

Examples

In the following example, the automatic exchange of IP version 4 unicast address family routing information is disabled:

Router(config)# router bgp 50000

Router(config-router)# no bgp default ipv4-unicast 

Related Commands

bgp default local-preference

To change the default local preference value, use the bgp default local-preference command in router configuration mode. To return the local preference value to the default setting, use the no form of this command.

bgp default local-preference number

no bgp default local-preference number

Syntax Description

Command Default

Cisco IOS software applies a local preference value of 100 if this command is not enabled or if the no form of this command is entered.

Command Modes

Router configuration

Command History

Usage Guidelines

The local preference attribute is a discretionary attribute that is used to apply the degree of preference to a route during the BGP best path selection process. This attribute is exchanged only between iBGP peers and is used to determine local policy. The route with the highest local preference is preferred.

Examples

In the following example, the local preference value is set to 200:

Router(config)# router bgp 50000 

Router(config-router)# bgp default local-preference 200 

Related Commands

bgp deterministic-med

To enforce the deterministic comparison of the Multi Exit Discriminator (MED) value between all paths received from within the same autonomous system, use the bgp deterministic-med command in router configuration mode. To disable the required MED comparison, use the no form of this command.

bgp deterministic-med

no bgp deterministic-med

Syntax Description

This command has no arguments or keywords.

Command Default

Cisco IOS software does not enforce the deterministic comparison of the MED variable between all paths received from the same autonomous system.

Command Modes

Router configuration

Command History

Usage Guidelines

The bgp always-compare-med command is used to enable the comparison of the Multi Exit Discriminator (MED) for paths from neighbors in different autonomous systems. After the bgp always-compare-med command is configured, all paths for the same prefix that are received from different neighbors, which are in the same autonomous system, will be grouped together and sorted by the ascending MED value (received-only paths are ignored and not grouped or sorted). The best path selection algorithm will then pick the best paths using the existing rules; the comparison is made on a per neighbor autonomous system basis and then global basis. The grouping and sorting of paths occurs immediately after this command is entered. For correct results, all routers in the local autonomous system must have this command enabled (or disabled).

Examples

In the following example, BGP is configured to compare the MED during path selection for routes advertised by the same subautonomous system within a confederation:

Router(config)# router bgp 50000 

Router(config-router)# bgp deterministic-med

The following example show ip bgp command output shows how route selection is affected by the configuration of the bgp deterministic-med command. The order in which routes are received affects how routes are selected for best path selection when the bgp deterministic-med command is not enabled. The following sample output from the show ip bgp command shows three paths that are received for the same prefix (10.100.0.0), and the bgp deterministic-med command is not enabled:

Router# show ip bgp 10.100.0.0 

BGP routing table entry for 10.100.0.0/16, version 40 

Paths: (3 available, best #3, advertised over IBGP, EBGP) 

  109 

    192.168.43.10 from 192.168.43.10 (192.168.43.1) 

      Origin IGP, metric 0, localpref 100, valid, internal 

  2051 

    192.168.43.22 from 192.168.43.22 (192.168.43.2) 

      Origin IGP, metric 20, localpref 100, valid, internal 

  2051 

    192.168.43.3 from 192.168.43.3 (10.4.1.1) 

      Origin IGP, metric 30, valid, external, best 

If the bgp deterministic-med feature is not enabled on the router, the route selection can be affected by the order in which the routes are received. Consider the following scenario in which a router received three paths for the same prefix:

The clear ip bgp * command is entered to clear all routes in the local routing table.

Router# clear ip bgp * 

The show ip bgp command is issued again after the routing table has been repopulated. Note that the order of the paths changed after clearing the BGP session. The results of the selection algorithm also changed because the order in which the paths were received was different for the second session.

Router# show ip bgp 10.100.0.0 

BGP routing table entry for 10.100.0.0/16, version 2 

Paths: (3 available, best #3, advertised over EBGP) 

  109 192.168.43.10 from 192.168.43.10 (192.168.43.1) 

      Origin IGP, metric 0, localpref 100, valid, internal 

  2051 

    192.168.43.3 from 192.168.43.3 (10.4.1.1) 

      Origin IGP, metric 30, valid, external 

  2051 

    192.168.43.22 from 192.168.43.22 (192.168.43.2) 

      Origin IGP, metric 20, localpref 100, valid, internal, best 

If the bgp deterministic-med command is enabled, then the result of the selection algorithm will always be the same, regardless of the order in which the paths are received by the local router. The following output is always generated when the bgp deterministic-med command is entered on the local router in this scenario:

Router# show ip bgp 10.100.0.0 

BGP routing table entry for 10.100.0.0/16, version 15 

Paths: (3 available, best #1, advertised over EBGP) 

  109 

    192.168.43.10 from 192.168.43.10 (192.168.43.1) 

      Origin IGP, metric 0, localpref 100, valid, internal, best 3 

    192.168.43.22 from 192.168.43.22 (192.168.43.2) 

      Origin IGP, metric 20, localpref 100, valid, internal 3 

    192.168.43.3 from 192.168.43.3 (10.4.1.1) 

      Origin IGP, metric 30, valid, external 

Related Commands

bgp dmzlink-bw

To configure BGP to distribute traffic proportionally over external links with unequal bandwidth when multipath load balancing is enabled, use the bgp dmzlink-bw command in address family configuration mode. To disable traffic distribution that is proportional to the link bandwidth, use the no form of this command.

bgp dmzlink-bw

no bgp dmzlink-bw

Syntax Description

This command has no arguments or keywords.

Command Default

BGP traffic is not distributed proportionally over external links with unequal bandwidth.

Command Modes

Address family configuration

Command History

Usage Guidelines

The bgp dmzlink-bw command is used to configure BGP to distribute traffic proportionally to the bandwidth of external links. This command is configured for multipath load balancing between directly connected external BGP (eBGP) neighbors. This command is used with BGP multipath features to configure load balancing over links with unequal bandwidth. The neighbor dmzlink-bw command must also be configured for each external link through which multipath load balancing is configured to advertise the link bandwidth as an extended community. The neighbor send-community command must be configured to exchange the link bandwidth extended community with internal BGP (iBGP) peers.

Examples

The following example shows how to configure the bgp dmzlink-bw command to allow multipath load balancing to distribute link traffic proportionally to the bandwidth of each external link and to advertise the bandwidth of these links to iBGP peers as an extended community:

Router(config)# router bgp 45000 

Router(config-router)# neighbor 10.10.10.1 remote-as 100 

Router(config-router)# neighbor 10.10.10.1 update-source Loopback 0 

Router(config-router)# neighbor 10.10.10.3 remote-as 100 

Router(config-router)# neighbor 10.10.10.3 update-source Loopback 0 

Router(config-router)# neighbor 172.16.1.1 remote-as 200 

Router(config-router)# neighbor 172.16.1.1 ebgp-multihop 1 

Router(config-router)# neighbor 172.16.2.2 remote-as 200 

Router(config-router)# neighbor 172.16.2.2 ebgp-multihop 1 

Router(config-router)# address-family ipv4 

Router(config-router-af)# bgp dmzlink-bw 

Router(config-router-af)# neighbor 10.10.10.1 activate 

Router(config-router-af)# neighbor 10.10.10.1 next-hop-self 

Router(config-router-af)# neighbor 10.10.10.1 send-community both 

Router(config-router-af)# neighbor 10.10.10.3 activate 

Router(config-router-af)# neighbor 10.10.10.3 next-hop-self 

Router(config-router-af)# neighbor 10.10.10.3 send-community both 

Router(config-router-af)# neighbor 172.16.1.1 activate 

Router(config-router-af)# neighbor 172.16.1.1 dmzlink-bw 

Router(config-router-af)# neighbor 172.16.2.2 activate 

Router(config-router-af)# neighbor 172.16.2.2 dmzlink-bw

Router(config-router-af)# maximum-paths ibgp 6

Router(config-router-af)# maximum-paths 6

Related Commands

bgp enforce-first-as

To configure a router to deny an update received from an external BGP (eBGP) peer that does not list its autonomous system number at the beginning of the AS_PATH in the incoming update, use the bgp enforce-first-as command in router configuration mode. To disable this behavior, use the no form of this command.

bgp enforce-first-as

no bgp enforce-first-as

Syntax Description

This command has no arguments or keywords.

Defaults

The behavior of this command is enabled by default.

Command Modes

Router configuration

Command History

Usage Guidelines

The bgp enforce-first-as command is used to deny incoming updates received from eBGP peers that do not list their autonomous system number as the first segment in the AS_PATH attribute. Enabling this command prevents a misconfigured or unauthorized peer from misdirecting traffic (spoofing the local router) by advertising a route as if it was sourced from another autonomous system.

Examples

In the following example, all incoming updates from eBGP peers are examined to ensure that the first autonomous system number in the AS_PATH is the local AS number of the transmitting peer. In the follow example, updates from the 10.100.0.1 peer will be discarded if the first AS number is not 65001.

Router(config)# router bgp 50000 

Router(config-router)# bgp enforce-first-as 

Router(config-router)# address-family ipv4 

Router(config-router-af)# neighbor 10.100.0.1 remote-as 65001

Router(config-router-af)# end 

bgp fast-external-fallover

To configure a Border Gateway Protocol (BGP) routing process to immediately reset external BGP peering sessions if the link used to reach these peers goes down, use the bgp fast-external-fallover command in router configuration mode. To disable BGP fast external fallover, use the no form of this command.

bgp fast-external-fallover

no bgp fast-external-fallover

Syntax Description

This command has no arguments or keywords.

Command Default

BGP fast external fallover is enabled by default in Cisco IOS software.

Command Modes

Router configuration

Command History

Usage Guidelines

The bgp fast-external-fallover command is used to disable or enable fast external fallover for BGP peering sessions with directly connected external peers. The session is immediately reset if link goes down. Only directly connected peering sessions are supported.

If BGP fast external fallover is disabled, the BGP routing process will wait until the default hold timer expires (3 keepalives) to reset the peering session. BGP fast external fallover can also be configured on a per-interface basis using the ip bgp fast-external-fallover interface configuration command.

Examples

In the following example, the BGP fast external fallover feature is disabled. If the link through which this session is carried flaps, the connection will not be reset.

Router(config)# router bgp 50000 

Router(config-router)# no bgp fast-external-fallover 

Related Commands

bgp graceful-restart

To enable the Border Gateway Protocol (BGP) graceful restart capability globally for all BGP neighbors, use the bgp graceful-restart command in address family or in router configuration mode. To disable the BGP graceful restart capability globally for all BGP neighbors, use the no form of this command.

bgp graceful-restart [restart-time seconds | stalepath-time seconds] [all]

no bgp graceful-restart

Syntax Description

Command Default

The following default values are used when this command is entered without any keywords or arguments:

restart-time: 120 seconds
stalepath-time: 360 seconds

Note Changing the restart and stalepath timer values is not required to enable the BGP graceful restart capability. The default values are optimal for most network deployments, and these values should be adjusted only by an experienced network operator.

Command Modes

Address-family configuration (config-router-af)
Router configuration (router-config)

Command History

Usage Guidelines

The bgp graceful-restart command is used to enable or disable the graceful restart capability globally for all BGP neighbors in a BGP network. The graceful restart capability is negotiated between nonstop forwarding (NSF)-capable and NSF-aware peers in OPEN messages during session establishment. If the graceful restart capability is enabled after a BGP session has been established, the session will need to be restarted with a soft or hard reset.

The graceful restart capability is supported by NSF-capable and NSF-aware routers. A router that is NSF-capable can perform a stateful switchover (SSO) operation (graceful restart) and can assist restarting peers by holding routing table information during the SSO operation. A router that is NSF-aware functions like a router that is NSF-capable but cannot perform an SSO operation.

The BGP graceful restart capability is enabled by default when a supporting version of Cisco IOS software is installed. The default timer values for this feature are optimal for most network deployments. We recommend that they are adjusted only by experienced network operators. When adjusting the timer values, the restart timer should not be set to a value greater than the hold time that is carried in the OPEN message. If consecutive restart operations occur, routes (from a restarting router) that were previously marked as stale will be deleted.

Note Changing the restart and stalepath timer values is not required to enable the BGP graceful restart capability. The default values are optimal for most network deployments, and these values should be adjusted only by an experienced network operator.

Examples

In the following example, the BGP graceful restart capability is enabled:

Router# configure terminal

Router(config)# router bgp 65000

Router(config-router)# bgp graceful-restart

In the following example, the restart timer is set to 130 seconds:

Router# configure terminal

Router(config)# router bgp 65000

Router(config-router)# bgp graceful-restart restart-time 130 

In the following example, the stalepath timer is set to 350 seconds:

Router# configure terminal

Router(config)# router bgp 65000

Router(config-router)# bgp graceful-restart stalepath-time 350

Related Commands

bgp inject-map

To configure conditional route injection to inject more specific routes into a Border Gateway Protocol (BGP) routing table, use the bgp inject-map command in address family or router configuration mode. To disable a conditional route injection configuration, use the no form of this command.

bgp inject-map inject-map exist-map exist-map [copy-attributes]

no bgp inject-map inject-map exist-map exist-map

Syntax Description

Command Default

No specific routes are injected into a BGP routing table.

Command Modes

Address family configuration
Router configuration

Command History

Usage Guidelines

The bgp inject-map command is used to configure conditional route injection. Conditional route injection allows you to originate a more specific prefix into a BGP routing table without a corresponding match. Two route maps (exist-map and inject-map) are configured in global configuration mode and then specified with the bgp inject-map command in address family or router configuration mode.

The exist-map argument specifies a route map that defines the prefix that the BGP speaker will track. This route map must contain a match ip address prefix-list command statement to specify the aggregate prefix and a match ip route-source prefix-list command statement to specify the route source.

The inject-map argument defines the prefixes that will be created and installed into the routing table. Injected prefixes are installed in the local BGP RIB. A valid parent route must exist; Only prefixes that are equal to or more specific than the aggregate route (existing prefix) can be injected.

The optional copy-attributes keyword is used to optionally configure the injected prefix to inherit the same attributes as the aggregate route. If this keyword is not entered, the injected prefix will use the default attributes for locally originated routes.

Examples

In the following example, conditional route injection is configured. Injected prefixes will inherit the attributes of the aggregate (parent) route.

Router(config)# ip prefix-list ROUTE permit 10.1.1.0/24 

Router(config)# ip prefix-list ROUTE_SOURCE permit 10.2.1.1/32

Router(config)# ip prefix-list ORIGINATED_ROUTES permit 10.1.1.0/25 

Router(config)# ip prefix-list ORIGINATED_ROUTES permit 10.1.1.128/25

Router(config)# route-map LEARNED_PATH permit 10 

Router(config-route-map)# match ip address prefix-list ROUTE 

Router(config-route-map)# match ip route-source prefix-list ROUTE_SOURCE

Router(config-route-map)# exit 

Router(config)# route-map ORIGINATE permit 10 

Router(config-route-map)# set ip address prefix-list ORIGINATED_ROUTES

Router(config-route-map)# set community 14616:555 additive 

Router(config-route-map)# exit 

Router(config)# router bgp 50000 

Router(config-router)# address-family ipv4 

Router(config-router-af)# bgp inject-map ORIGINATE exist-map LEARNED_PATH copy-attributes

Router(config-router-af)# end 

Related Commands

bgp listen

To associate a subnet range with a Border Gateway Protocol (BGP) peer group and activate the BGP dynamic neighbors feature, use the bgp listen command in router configuration mode. To disable the BGP dynamic neighbors feature, use the no form of this command.

bgp listen [limit max-number | range network/length peer-group peer-group-name]

no bgp listen [limit | range network/length peer-group peer-group-name]

Syntax Description

Command Default

No subnets are associated with a BGP listen range group, and the BGP dynamic neighbor feature is not activated.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

Use the limit keyword and max-number argument to define the global maximum number of BGP dynamic neighbors that can be created.

BGP dynamic neighbors are configured using a range of IP addresses and BGP peer groups. Each range can be configured as a subnet IP address. After a subnet range is configured for a BGP peer group, and a TCP session is initiated for an IP address in the subnet range, a new BGP neighbor is dynamically created as a member of that group. The new BGP neighbor will inherit any configuration for the peer group. Only IPv4 peering is supported. The output for three show commands has been updated to display information about dynamic neighbors. The commands are show ip bgp neighbors, show ip bgp peer-group, and the show ip bgp summary command.

Examples

The following example configures a subnet range of 192.168.0.0/16 and associates this listen range with a BGP peer group. Note that the listen range peer group that is configured for the BGP dynamic neighbor feature can be activated in the IPv4 address family using the neighbor activate command. After the initial configuration on Router 1, when Router 2 starts a BGP router session and adds Router 1 to its BGP neighbor table, a TCP session is initiated and Router 1 creates a new BGP neighbor dynamically because the IP address of the new neighbor is within the listen range subnet.

Router 1

enable

configure terminal

router bgp 45000

 bgp log-neighbor-changes

 neighbor group192 peer-group 

 bgp listen range 192.168.0.0/16 peer-group group192

 neighbor group192 ebgp-multihop 255

 neighbor group192 remote-as 40000 alternate-as 50000

 address-family ipv4 unicast 

 neighbor group192 activate

 end

Router 2

enable

configure terminal

router bgp 50000

 neighbor 192.168.3.1 remote-as 45000

 exit

If the show ip bgp summary command is now entered on Router 1, the output shows the dynamically 
created BGP neighbor, 192.168.3.2.

Router1# show ip bgp summary

BGP router identifier 192.168.3.1, local AS number 45000

BGP table version is 1, main routing table version 1

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

*192.168.3.2    4 50000       2       2        0    0    0 00:00:37        0

* Dynamically created based on a listen range command

Dynamically created neighbors: 1/(100 max), Subnet ranges: 1

BGP peergroup group192 listen range group members: 

  192.168.0.0/16 

Related Commands

bgp log-neighbor-changes

To enable logging of BGP neighbor resets, use the bgp log-neighbor-changes command in router configuration mode. To disable the logging of changes in BGP neighbor adjacencies, use the no form of this command.

bgp log-neighbor-changes

no bgp log-neighbor-changes

Syntax Description

This command has no arguments or keywords.

Command Default

Logging of BGP neighbor resets is not enabled.

Command Modes

Router configuration

Command History

Usage Guidelines

The bgp log-neighbor-changes command enables logging of BGP neighbor status changes (up or down) and resets for troubleshooting network connectivity problems and measuring network stability. Unexpected neighbor resets might indicate high error rates or high packet loss in the network and should be investigated.

Using the bgp log-neighbor-changes command to enable status change message logging does not cause a substantial performance impact, unlike, for example, enabling per BGP update debugging. If the UNIX syslog facility is enabled, messages are sent to the UNIX host running the syslog daemon so that the messages can be stored and archived. If the UNIX syslog facility is not enabled, the status change messages are retained in the internal buffer of the router, and are not stored to disk. You can set the size of this buffer, which is dependent upon the available RAM, using the logging buffered command.

The neighbor status change messages are not tracked if the bgp log-neighbor-changes command is not enabled, except for the reset reason, which is always available as output of the show ip bgp neighbors and show bgp ipv6 neighbors commands.

The eigrp log-neighbor-changes command enables logging of Enhanced Interior Gateway Routing Protocol (EIGRP) neighbor adjacencies, but messages for BGP neighbors are logged only if they are specifically enabled with the bgp log-neighbor-changes command.

Use the show logging command to display the log for the BGP neighbor changes.

Examples

The following example logs neighbor changes for BGP in router configuration mode:

Router(config)# bgp router 40000 

Router(config-router)# bgp log-neighbor-changes

Related Commands

bgp maxas-limit

To configure Border Gateway Protocol (BGP) to discard routes that have a number of autonomous system numbers in AS-path that exceed the specified value, use the bgp maxas-limit command in router configuration mode. To return the router to default operation, use the no form of this command.

bgp maxas-limit number

no bgp maxas-limit

Syntax Description

Command Default

No routes are discarded.

Command Modes

Router configuration

Command History

Usage Guidelines

The bgp maxas-limit command is used to limit the number of autonomous system numbers in the AS-path attribute that are permitted in inbound routes. If a route is received with an AS-path segment that exceeds the configured limit, the BGP routing process will discard the route.

Examples

This example sets a maximum number of autonomous systems numbers in the AS-path attribute to 30:

Router(config)# router bgp 40000

Router(config-router-af)# bgp maxas-limit 30 

Related Commands

bgp nexthop

To configure Border Gateway Protocol (BGP) next-hop address tracking, use the bgp nexthop command in address family or router configuration mode. To disable BGP next-hop address tracking, use the no form of this command.

bgp nexthop {trigger {delay seconds | enable} | route-map map-name}

no bgp nexthop {trigger {delay | enable} | route-map map-name}

Syntax Description

Command Default

BGP next-hop address tracking is enabled by default for IPv4 and VPNv4 address families. It is also enabled by default for the VPNv6 address family as of Cisco IOS Release 12.2(33)SB6.

Command Modes

Address family configuration
Router configuration

Command History

Usage Guidelines

BGP next-hop address tracking is event driven. BGP prefixes are automatically tracked as peering sessions are established. Next-hop changes are rapidly reported to BGP as they are updated in the routing information base (RIB). This optimization improves overall BGP convergence by reducing the response time to next-hop changes for routes installed in the RIB. When a best-path calculation is run in between BGP scanner cycles, only the changes are processed and tracked.

Note BGP next-hop address tracking improves BGP response time significantly. However, unstable Interior Gateway Protocol (IGP) peers can introduce instability to BGP. We recommend that you aggressively dampen unstable IGP peering sessions to mitigate the possible impact to BGP.

Note BGP next-hop address tracking is not supported under the IPv6 address family.

Use the trigger keyword with the delay keyword and seconds argument to change the delay interval between routing table walks for BGP next-hop address tracking. You can increase the performance of BGP next-hop address tracking by tuning the delay interval between full routing table walks to match the tuning parameters for the IGP. The default delay interval is 5 seconds, which is an optimal value for a fast-tuned IGP. In the case of an IGP that converges more slowly, you can change the delay interval to 20 seconds or more, depending on the IGP convergence time.

Use the trigger keyword with the enable keyword to enable BGP next-hop address tracking. BGP next-hop address tracking is enabled by default.

Use the route-map keyword and map-name argument to allow a route map to be used. The route map is used during the BGP best-path calculation and is applied to the route in the routing table that covers the Next_Hop attribute for BGP prefixes. If the next-hop route fails the route-map evaluation, the next-hop route is marked as unreachable. This command is per address family, so different route maps can be applied for next-hop routes in different address families.

Note Only the match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported.

Examples

The following example shows how to change the delay interval between routing table walks for BGP next-hop address tracking to occur every 20 seconds under an IPv4 address family session:

router bgp 50000 

 address-family ipv4 unicast 

 bgp nexthop trigger delay 20 

 end

The following example shows how to disable next-hop address tracking for the IPv4 address family:

router bgp 50000 

 address-family ipv4 unicast

 no bgp nexthop trigger enable

 end

The following example shows how to configure a route map that permits a route to be considered as a next-hop route only if the address mask length is more than 25. This configuration will avoid any prefix aggregates being considered as a next-hop route.

router bgp 45000 

 address-family ipv4 unicast

 bgp nexthop route-map CHECK-NEXTHOP

 exit-address-family

 exit

ip prefix-list FILTER25 seq 5 permit 0.0.0.0/0 ge 25

route-map CHECK-NEXTHOP permit 10

 match ip address prefix-list FILTER25

 end

Related Commands

bgp nexthop trigger delay

The trigger and delay keywords for the bgp nexthop command are no longer documented as a separate command.

The information for using the trigger and delay keywords for the bgp nexthop command has been incorporated into the bgp nexthop command documentation. See the bgp nexthop command documentation for more information.

bgp nexthop trigger enable

The trigger and enable keywords for the bgp nexthop command are no longer documented as a separate command.

The information for using the trigger and enable keywords for the bgp nexthop command has been incorporated into the bgp nexthop command documentation. See the bgp nexthop command documentation for more information.

bgp nopeerup-delay

To configure the time duration that Border Gateway Protocol (BGP) waits for the first peer to come up before populating the routing information base (RIB), use the bgp nopeerup-delay command in router configuration mode. To remove the configured values, use the no form of this command.

bgp nopeerup-delay {cold-boot | nsf-switchover | post-boot | user-initiated} seconds

no bgp nopeerup-delay {cold-boot | nsf-switchover | post-boot | user-initiated} seconds

Syntax Description

Command Default

Delay time is not configured.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

In a Virtual Switching System (VSS), Open Shortest Path First (OSPF) NSF Engineering Task Force (IETF) operations and BGP are configured and peers are propagated through OSPF. In such a VSS, the OSPF restart interval should be shorter than the time BGP waits for the first peer to come up before populating the RIB; otherwise traffic will be dropped. To make the OSPF restart interval shorter than the time BGP waits for the first peer to come up, use the nsf ietf restart-interval command. To change the time duration that BGP waits for the first peer to come up, and make it longer than the OSPF restart interval, use the bgp nopeerup-delay command.

Examples

The following example shows how to configure the delay time to 234 seconds for the first peer to come up after NSF switchover.

Router(config)# router bgp 100

Router(config-router)# bgp nopeerup-delay nsf-switchover 234 

Related Commands

bgp recursion host

To enable the recursive-via-host flag for IP Version 4 (IPv4), Virtual Private Network (VPN) Version 4 (VPNv4), Virtual Routing and Forwarding (VRF) address families, and IPv6 address families, use the bgp recursion host command in address family configuration or router configuration mode. To disable the recursive-via-host flag, use the no form of this command.

bgp recursion host

no bgp recursion host

Syntax Description

This command has no arguments or keywords.

Command Default

For an internal Border Gateway Protocol (iBGP) IPv4 address family, irrespective of whether Prefix Independent Convergence (PIC) is enabled, the recursive-via-host flag in Cisco Express Forwarding is not set.

For the VPNv4 and IPv4 VRF address families, the recursive-via-host flag is set and the bgp recursion host command is automatically restored when PIC is enabled under the following conditions:

•The bgp additional-paths install command is enabled.

•The bgp advertise-best-external command is enabled.