IP Routing: BGP Configuration Guide, Cisco IOS Release 15M&T
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This module describes how to configure internal Border Gateway Protocol (BGP) features. Internal BGP (iBGP) refers to running
BGP on networking devices within one autonomous system. BGP is an interdomain routing protocol designed to provide loop-free
routing between separate routing domains (autonomous systems) that contain independent routing policies. Many companies now
have large internal networks, and there are many issues involved in scaling the existing internal routing protocols to match
the increasing traffic demands while maintaining network efficiency.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information,
see
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and to see a list of the releases in which each feature is supported, see the feature information table at the end of this
module.
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Information About Internal BGP Features
BGP Routing Domain Confederation
One way to reduce the internal BGP (iBGP) mesh is to divide an autonomous system into multiple subautonomous systems and
group them into a single confederation. To the outside world, the confederation looks like a single autonomous system. Each
autonomous system is fully meshed within itself and has a few connections to other autonomous systems in the same confederation.
Even though the peers in different autonomous systems have external BGP (eBGP) sessions, they exchange routing information
as if they were iBGP peers. Specifically, the next hop, Multi Exit Discriminator (MED) attribute, and local preference information
are preserved. This feature allows the you to retain a single Interior Gateway Protocol (IGP) for all of the autonomous systems.
To configure a BGP confederation, you must specify a confederation identifier. To the outside world, the group of autonomous
systems will look like a single autonomous system with the confederation identifier as the autonomous system number.
BGP Route Reflector
BGP requires that all iBGP speakers be fully meshed. However, this requirement does not scale well when there are many iBGP
speakers. Instead of configuring a confederation, another way to reduce the iBGP mesh is to configure a route reflector.
The figure below illustrates a simple iBGP configuration with three iBGP speakers (Routers A, B, and C). Without route reflectors,
when Router A receives a route from an external neighbor, it must advertise it to both routers B and C. Routers B and C do
not readvertise the iBGP learned route to other iBGP speakers because the routers do not pass on routes learned from internal
neighbors to other internal neighbors, thus preventing a routing information loop.
Figure 1. Three Fully Meshed iBGP Speakers
With route reflectors, all iBGP speakers need not be fully meshed because there is a method to pass learned routes to neighbors.
In this model, an iBGP peer is configured to be a route reflector responsible for passing iBGP learned routes to a set of
iBGP neighbors. In the figure below, Router B is configured as a route reflector. When the route reflector receives routes
advertised from Router A, it advertises them to Router C, and vice versa. This scheme eliminates the need for the iBGP session
between Routers A and C.
Figure 2. Simple BGP Model with a Route Reflector
The internal peers of the route reflector are divided into two groups: client peers and all the other routers in the autonomous
system (nonclient peers). A route reflector reflects routes between these two groups. The route reflector and its client peers
form a
cluster. The nonclient peers must be fully meshed with each other, but the client peers need not be fully meshed. The clients in
the cluster do not communicate with iBGP speakers outside their cluster.
The figure below illustrates a more complex route reflector scheme. Router A is the route reflector in a cluster with routers
B, C, and D. Routers E, F, and G are fully meshed, nonclient routers.
Figure 3. More Complex BGP Route Reflector Model
When the route reflector receives an advertised route, depending on the neighbor, it takes the following actions:
A route from an external BGP speaker is advertised to all clients and nonclient peers.
A route from a nonclient peer is advertised to all clients.
A route from a client is advertised to all clients and nonclient peers. Hence, the clients need not be fully meshed.
Along with route reflector-aware BGP speakers, it is possible to have BGP speakers that do not understand the concept of
route reflectors. They can be members of either client or nonclient groups allowing an easy and gradual migration from the
old BGP model to the route reflector model. Initially, you could create a single cluster with a route reflector and a few
clients. All the other iBGP speakers could be nonclient peers to the route reflector and then more clusters could be created
gradually.
An autonomous system can have multiple route reflectors. A route reflector treats other route reflectors just like other
iBGP speakers. A route reflector can be configured to have other route reflectors in a client group or nonclient group. In
a simple configuration, the backbone could be divided into many clusters. Each route reflector would be configured with other
route reflectors as nonclient peers (thus, all the route reflectors will be fully meshed). The clients are configured to maintain
iBGP sessions with only the route reflector in their cluster.
Usually a cluster of clients will have a single route reflector. In that case, the cluster is identified by the router ID
of the route reflector. To increase redundancy and avoid a single point of failure, a cluster might have more than one route
reflector. In this case, all route reflectors in the cluster must be configured with the 4-byte cluster ID so that a route
reflector can recognize updates from route reflectors in the same cluster. All the route reflectors serving a cluster should
be fully meshed and all of them should have identical sets of client and nonclient peers.
Route Reflector Mechanisms to Avoid Routing Loops
As the iBGP learned routes are reflected, routing information may loop. The route reflector model has the following mechanisms
to avoid routing loops:
Originator ID is an optional, nontransitive BGP attribute. It is a 4-byte attribute created by a route reflector. The attribute
carries the router ID of the originator of the route in the local autonomous system. Therefore, if a misconfiguration causes
routing information to come back to the originator, the information is ignored.
Cluster-list is an optional, nontransitive BGP attribute. It is a sequence of cluster IDs that the route has passed. When
a route reflector reflects a route from its clients to nonclient peers, and vice versa, it appends the local cluster ID to
the cluster list. If the cluster list is empty, a new cluster list is created. Using this attribute, a route reflector can
identify if routing information is looped back to the same cluster due to misconfiguration. If the local cluster ID is found
in the cluster list, the advertisement is ignored.
The use of set clauses in outbound route maps can modify attributes and possibly create routing loops. To avoid this behavior, most set clauses of outbound route maps are ignored for routes reflected to iBGP peers. The only set clause of an outbound route map that is acted upon is the setipnext-hop clause.
BGP Outbound Route Map on Route Reflector to Set IP Next Hop for iBGP Peer
The BGP Outbound Route Map on Route Reflector to Set IP Next Hop feature allows a route reflector to modify the next hop
attribute for a reflected route.
The use of
set clauses in outbound route maps can modify attributes and possibly create routing loops. To avoid this behavior, most
set clauses of outbound route maps are ignored for routes reflected to iBGP peers. The only
set clause of an outbound route map on a route reflector (RR) that is acted upon is the
setipnext-hop clause. The
setipnext-hop clause is applied to reflected routes.
Configuring an RR with an outbound route map allows a network administrator to modify the next hop attribute for a reflected
route. By configuring a route map with the
setipnext-hop clause, the administrator puts the RR into the forwarding path, and can configure iBGP multipath load sharing to achieve
load balancing. That is, the RR can distribute outgoing packets among multiple egress points. See the “Configuring iBGP Multipath
Load Sharing” module.
Caution
Incorrectly setting BGP attributes for reflected routes can cause inconsistent routing, routing loops, or a loss of connectivity.
Setting BGP attributes for reflected routes should be attempted only by someone who has a good understanding of the design
implications.
BGP Route Dampening
Route dampening is a BGP feature designed to minimize the propagation of flapping routes across an internetwork. A route is
considered to be flapping when its availability alternates repeatedly.
For example, consider a network with three BGP autonomous systems: autonomous system 1, autonomous system 2, and autonomous
system 3. Suppose the route to network A in autonomous system 1 flaps (it becomes unavailable). Under circumstances without
route dampening, the eBGP neighbor of autonomous system 1 to autonomous system 2 sends a withdraw message to autonomous system
2. The border router in autonomous system 2, in turn, propagates the withdraw message to autonomous system 3. When the route
to network A reappears, autonomous system 1 sends an advertisement message to autonomous system 2, which sends it to autonomous
system 3. If the route to network A repeatedly becomes unavailable, then available, many withdrawal and advertisement messages
are sent. This is a problem in an internetwork connected to the Internet because a route flap in the Internet backbone usually
involves many routes.
Note
No penalty is applied to a BGP peer reset when route dampening is enabled. Although the reset withdraws the route, no penalty
is applied in this instance, even if route flap dampening is enabled.
Route Dampening Minimizes Route Flapping
The route dampening feature minimizes the flapping problem as follows. Suppose again that the route to network A flaps. The
router in autonomous system 2 (where route dampening is enabled) assigns network A a penalty of 1000 and moves it to history
state. The router in autonomous system 2 continues to advertise the status of the route to neighbors. The penalties are cumulative.
When the route flaps so often that the penalty exceeds a configurable suppress limit, the router stops advertising the route
to network A, regardless of how many times it flaps. Thus, the route is dampened.
The penalty placed on network A is decayed until the reuse limit is reached, upon which the route is once again advertised.
At half of the reuse limit, the dampening information for the route to network A is removed.
BGP Route Dampening Terms
The following terms are used when describing route dampening:
History state—After a route flaps once, it is assigned a penalty and put into history state, meaning the router does not
have the best path, based on historical information.
Penalty—Each time a route flaps, the router configured for route dampening in another autonomous system assigns the route
a penalty of 1000. Penalties are cumulative. The penalty for the route is stored in the BGP routing table until the penalty
exceeds the suppress limit. At that point, the route state changes from history to damp.
Damp state—In this state, the route has flapped so often that the router will not advertise this route to BGP neighbors.
Suppress limit—A route is suppressed when its penalty exceeds this limit. The default value is 2000.
Half-life—Once the route has been assigned a penalty, the penalty is decreased by half after the half-life period (which
is 15 minutes by default). The process of reducing the penalty happens every 5 seconds.
Reuse limit—As the penalty for a flapping route decreases and falls below this reuse limit, the route is unsuppressed. That
is, the route is added back to the BGP table and once again used for forwarding. The default reuse limit is 750. The process
of unsuppressing routes occurs at 10-second increments. Every 10 seconds, the router finds out which routes are now unsuppressed
and advertises them to the world.
Maximum suppress limit—This value is the maximum amount of time a route can be suppressed. The default value is four times
the half-life.
The routes external to an autonomous system learned via iBGP are not dampened. This policy prevent the iBGP peers from having
a higher penalty for routes external to the autonomous system.
BGP Route Map Next Hop Self
The BGP Route Map Next Hop Self feature provides a way to override the settings for bgp next-hop unchanged and bgp next-hop
unchanged allpath selectively. These settings are global for an address family. For some routes this may not be appropriate.
For example, static routes may need to be redistributed with a next hop of self, but connected routes and routes learned
via Interior Border Gateway Protocol (IBGP) or Exterior Border Gateway Protocol (EBGP) may continue to be redistributed with
an unchanged next hop.
The BGP route map next hop self functionality modifies the existing route map infrastructure to configure a new ip next-hop
self setting, which overrides the bgp next-hop unchanged and bgp next-hop unchanged allpaths settings.
The ip next-hop self setting is applicable only to VPNv4 and VPNv6 address families. Routes distributed by protocols other
than BGP are not affected.
You configure a new bgp route-map priority setting to inform BGP that the route map will take priority over the settings
for bgp next-hop unchanged and bgp next-hop unchanged allpath. The bgp route-map priority setting only impacts BGP. The bgp
route-map priority setting has no impact unless you configure the bgp next-hop unchanged or bgp next-hop unchanged allpaths
settings.
How to Configure Internal BGP Features
Configuring a Routing Domain Confederation
To configure a BGP confederation, you must specify a confederation identifier. To the outside world, the group of autonomous
systems will look like a single autonomous system with the confederation identifier as the autonomous system number. To configure
a BGP confederation identifier, use the following command in router configuration mode:
In order to treat the neighbors from other autonomous systems within the confederation as special eBGP peers, use the following
command in router configuration mode:
Configures the local router as a BGP route reflector and the specified neighbor as a client.
If the cluster has more than one route reflector, configure the cluster ID by using the following command in router configuration
mode:
Command
Purpose
Router(config-router)# bgp cluster-id cluster-id
Configures the cluster ID.
Use the showipbgp command to display the originator ID and the cluster-list attributes.
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, the route reflector need not reflect routes to clients.
To disable client-to-client route reflection, use the nobgpclient-to-clientreflection command in router configuration mode:
Command
Purpose
Router(config-router)# no bgp client-to-client reflection
Disables client-to-client route reflection.
Configuring a Route Reflector Using a Route Map to a Set Next Hop for an iBGP Peer
Perform this task on an RR to set a next hop for an iBGP peer. One reason to perform this task is when you want to make the
RR the next hop for routes, so that you can configure iBGP load sharing. Create a route map that sets the next hop to be the
RR’s address, which will be advertised to the RR clients. The route map is applied only to outbound routes from the router
to which the route map is applied.
Caution
Incorrectly setting BGP attributes for reflected routes can cause inconsistent routing, routing loops, or a loss of connectivity.
Setting BGP attributes for reflected routes should only be attempted by someone who has a good understanding of the design
implications.
Note
Do not use the
neighbornext-hop-self command to modify the next hop attribute for an RR. Using the
neighbornext-hop-self command on the RR will modify next hop attributes only for non-reflected routes and not the intended routes that are being
reflected from the RR clients. To modify the next hop attribute when reflecting a route, use an outbound route map.
This task configures the RR (Router 2) in the scenario illustrated in the figure below. In this case, Router 1 is the iBGP
peer whose routes’ next hop is being set. Without a route map, outbound routes from Router 1 would go to next hop Router 3.
Instead, setting the next hop to the RR’s address will cause routes from Router 1 to go to the RR, and thus allow the RR to
perform load balancing among Routers 3, 4, and 5.
Figure 4. Route Reflector Using a Route Map to a Set Next Hop for an iBGP Peer
SUMMARY STEPS
enable
configureterminal
route-mapmap-tag
setipnext-hopip-address
exit
routerbgpas-number
address-familyipv4
maximum-pathsibgpnumber
neighborip-addressremote-asas-number
neighborip-addressactivate
neighborip-addressroute-reflector-client
neighborip-addressroute-mapmap-nameout
Repeat Steps 12 through 14 for the other RR clients.
end
showipbgpneighbors
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Router> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
configureterminal
Example:
Router# configure terminal
Enters global configuration mode.
Step 3
route-mapmap-tag
Example:
Router(config)# route-map rr-out
Enters route map configuration mode to configure a route map.
The route map is created to set the next hop for the route reflector client.
Step 4
setipnext-hopip-address
Example:
Router(config-route-map)# set ip next-hop 10.2.0.1
Specifies that for routes that are advertised where this route map is applied, the next-hop attribute is set to this IPv4
address.
For this task, we want to set the next hop to be the address of the RR.
Step 5
exit
Example:
Router(config-route-map)# exit
Exits route-map configuration mode and enters global configuration mode.
Step 6
routerbgpas-number
Example:
Router(config)# router bgp 100
Enters router configuration mode and creates a BGP routing process.
Step 7
address-familyipv4
Example:
Router(config-router-af)# address-family ipv4
Enters address family configuration mode to configure BGP peers to accept address family specific configurations.
Step 8
maximum-pathsibgpnumber
Example:
Router(config-router)# maximum-paths ibgp 5
Controls the maximum number of parallel iBGP routes that can be installed in the routing table.
Configures the local router as a BGP route reflector, and configures the specified neighbor as a route-reflector client.
Step 12
neighborip-addressroute-mapmap-nameout
Example:
Router(config-router-af)# neighbor 10.1.0.1 route-map rr-out out
Applies the route map to outgoing routes from this neighbor.
Reference the route map you created in Step 3.
Step 13
Repeat Steps 12 through 14 for the other RR clients.
You will not be applying a route map to the other RR clients.
Step 14
end
Example:
Router(config-router-af)# end
Exits address family configuration mode and enters privileged EXEC mode.
Step 15
showipbgpneighbors
Example:
Router# show ip bgp neighbors
(Optional) Displays information about the BGP neighbors, including their status as RR clients, and information about the
route map configured.
Adjusting BGP Timers
BGP uses certain timers to control periodic activities such as the sending of keepalive messages and the interval after not
receiving a keepalive message after which the Cisco software declares a peer dead. By default, the keepalive timer is 60 seconds,
and the hold-time timer is 180 seconds. You can adjust these timers. When a connection is started, BGP will negotiate the
hold time with the neighbor. The smaller of the two hold times will be chosen. The keepalive timer is then set based on the
negotiated hold time and the configured keepalive time.
To adjust BGP timers for all neighbors, use the following command in router configuration mode:
Sets the keepalive and hold-time timers (in seconds) for the specified peer or peer group.
Note
The timers configured for a specific neighbor or peer group override the timers configured for all BGP neighbors using the
timersbgp router configuration command.
To clear the timers for a BGP neighbor or peer group, use the
no form of the
neighbortimers command.
Configuring the Router to Consider a Missing MED as the Worst Path
To configure the router to consider a path with a missing MED attribute as the worst path, use the following command in router
configuration mode:
Configures the router to consider a missing MED as having a value of infinity, making the path without a MED value the least
desirable path.
Configuring the Router to Consider the MED to Choose a Path from Subautonomous System Paths
To configure the router to consider the MED value in choosing a path, use the following command in router configuration mode:
Command
Purpose
Router(config-router)# bgpbestpathmedconfed
Configures the router to consider the MED in choosing a path from among those advertised by different subautonomous systems
within a confederation.
The comparison between MEDs is made only if there are no external autonomous systems in the path (an external autonomous
system is an autonomous system that is not within the confederation). If there is an external autonomous system in the path,
then the external MED is passed transparently through the confederation, and the comparison is not made.
The following example compares route A with these paths:
In this case, path 1 would be chosen if the
bgpbestpathmedconfedrouterconfiguration command is enabled. The fourth path has a lower MED, but it is not involved in the MED comparison because there is an external
autonomous system is in this path.
Configuring the Router to Use the MED to Choose a Path in a Confederation
To configure the router to use the MED to choose the best path from among paths advertised by a single subautonomous system
within a confederation, use the following command in router configuration mode:
Command
Purpose
Router(config-router)# bgp deterministic med
Configures the router to compare the MED variable when choosing among routes advertised by different peers in the same autonomous
system.
Note
If the bgpalways-compare-med router configuration command is enabled, all paths are fully comparable, including those from other autonomous systems in
the confederation, even if the bgpdeterministicmed command is also enabled.
Configuring BGP Route Dampening
The tasks in this section configure and monitor BGP route dampening. Route dampening is designed to minimize the propagation
of flapping routes across an internetwork. A route is considered to be flapping when its availability alternates repeatedly.
Enabling and Configuring BGP Route Dampening
Perform this task to enable and configure BGP route dampening.
Specifies the IPv4 address family and enters address family configuration mode.
The
unicast keyword specifies the IPv4 unicast address family. By default, the router is placed in address family configuration mode
for the IPv4 unicast address family if the
unicast keyword is not specified with the
address-familyipv4 command.
The
multicast keyword specifies IPv4 multicast address prefixes.
The
vrf keyword and
vrf-name argument specify the name of the VRF instance to associate with subsequent IPv4 address family configuration mode commands.
Enables BGP route dampening and changes the default values of route dampening factors.
The
half-life,
reuse,
suppress, and
max-suppress-time arguments are all position dependent; if one argument is entered then all the arguments must be entered.
Use the
route-map keyword and
map-name argument to control where BGP route dampening is enabled.
Step 6
end
Example:
Router(config-router-af)# end
Exits address family configuration mode and enters privileged EXEC mode.
Monitoring and Maintaining BGP Route Dampening
Perform the steps in this task as required to monitor and maintain BGP route dampening.
Use this command to monitor the flaps of all the paths that are flapping. The statistics will be deleted once the route is
not suppressed and is stable for at least one half-life.
Example:
Router# show ip bgp dampening flap-statistics
BGP table version is 10, local router ID is 172.17.232.182
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network From Flaps Duration Reuse Path
*d 10.0.0.0 172.17.232.177 4 00:13:31 00:18:10 100
*d 10.2.0.0 172.17.232.177 4 00:02:45 00:28:20 100
Use this command to clear the accumulated penalty for routes that are received on a router that has BGP dampening enabled.
If no arguments or keywords are specified, flap statistics are cleared for all routes. Flap statistics are also cleared when
the peer is stable for the half-life time period. After the BGP flap statistics are cleared, the route is less likely to be
dampened.
Example:
Router# clear ip bgp flap-statistics 172.17.232.177
Step 4
showipbgp
dampeningdampened-paths
Use this command to monitor the flaps of all the paths that are flapping. The statistics will be deleted once the route is
not suppressed and is stable for at least one half-life.
Example:
Router# show ip bgp dampening dampened-paths
BGP table version is 10, local router ID is 172.29.232.182
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network From Reuse Path
*d 10.0.0.0 172.16.232.177 00:18:4 100 ?
*d 10.2.0.0 172.16.232.177 00:28:5 100 ?
Use this command to clear stored route dampening information. If no keywords or arguments are entered, route dampening information
for the entire routing table is cleared. The following example clears route dampening information for VPNv4 address family
prefixes from network 192.168.10.0/24, and unsuppresses its suppressed routes.
Example:
Router# clear ip bgp vpnv4 unicast dampening 192.168.10.0 255.255.255.0
Monitoring and Maintaining BGP Route Dampening
You can monitor the flaps of all the paths that are flapping. The statistics will be deleted once the route is not suppressed
and is stable for at least one half-life. To display flap statistics, use the following commands as needed:
Command
Purpose
Router# show ip bgpdampening flap-statistics
Displays BGP flap statistics for all paths.
Router# show ip bgp dampening flap-statisticsregexpregexp
Displays BGP flap statistics for all paths that match the regular expression.
Displays BGP flap statistics for more specific entries.
To clear BGP flap statistics (thus making it less likely that the route will be dampened), use the following commands as
needed:
Command
Purpose
Router# clearipbgpflap-statistics
Clears BGP flap statistics for all routes.
Router# clearipbgpflap-statisticsregexpregexp
Clears BGP flap statistics for all paths that match the regular expression.
Router# clearipbgpflap-statisticsfilter-listlist
Clears BGP flap statistics for all paths that pass the filter.
Router# clearipbgpflap-statisticsip-addressmask
Clears BGP flap statistics for a single entry.
Router# clearipbgpip-addressflap-statistics
Clears BGP flap statistics for all paths from a neighbor.
Note
The flap statistics for a route are also cleared when a BGP peer is reset. Although the reset withdraws the route, there
is no penalty applied in this instance, even if route flap dampening is enabled.
Once a route is dampened, you can display BGP route dampening information, including the time remaining before the dampened
routes will be unsuppressed. To display the information, use the following command:
Command
Purpose
Router# showip bgpdampeningdampened-paths
Displays the dampened routes, including the time remaining before they will be unsuppressed.
You can clear BGP route dampening information and unsuppress any suppressed routes by using the following command:
Clears route dampening information and unsuppresses the suppressed routes.
Configuring BGP Route Map next-hop self
Perform this task to modify the existing route map by adding the ip next-hop self setting and overriding the bgp next-hop
unchanged and bgp next-hop unchanged allpaths settings.
Specifies the IPv4 address family and enters address family configuration mode.
Step 20
bgproute-mappriority
Example:
Device(config-router-af)# bgp route-map priority
Configures the route map priority for the local BGP routing process
Step 21
redistributeprotocol
Example:
Device(config-router-af)# redistribute static
Redistributes routes from one routing domain into another routing
domain.
Step 22
redistributeprotocol
Example:
Device(config-router-af)# redistribute connected
Redistributes routes from one routing domain into another routing
domain.
Step 23
exit-address-family
Example:
Device(config-router-af)# exit address-family
Exits address family configuration mode and enters router configuration mode .
Step 24
end
Example:
Device(config-router)# end
Exits router configuration mode and enters privileged EXEC mode.
Configuration Examples for Internal BGP Features
Example: BGP Confederation Configurations with Route Maps
This section contains an example of the use of a BGP confederation configuration that includes BGP communities and route
maps. For more examples of how to configure a BGP confederation, see the “Example: BGP Confederation” section in this module
This example shows how BGP community attributes are used with a BGP confederation configuration to filter routes.
In this example, the route map named
set-community is applied to the outbound updates to neighbor 172.16.232.50 and the local-as community attribute is used to filter the routes.
The routes that pass access list 1 have the special community attribute value local-as. The remaining routes are advertised
normally. This special community value automatically prevents the advertisement of those routes by the BGP speakers outside
autonomous system 200.
router bgp 65000
network 10.0.1.0 route-map set-community
bgp confederation identifier 200
bgp confederation peers 65001
neighbor 172.16.232.50 remote-as 100
neighbor 172.16.233.2 remote-as 65001
!
route-map set-community permit 10
match ip address 1
set community local-as
!
Example: BGP Confederation
The following is a sample configuration that shows several peers in a confederation. The confederation consists of three
internal autonomous systems with autonomous system numbers 6001, 6002, and 6003. To the BGP speakers outside the confederation,
the confederation looks like a normal autonomous system with autonomous system number 500 (specified via the
bgpconfederationidentifier router configuration command).
In a BGP speaker in autonomous system 6001, the
bgpconfederationpeers router configuration command marks the peers from autonomous systems 6002 and 6003 as special eBGP peers. Hence peers 172.16.232.55
and 172.16.232.56 will get the local preference, next hop, and MED unmodified in the updates. The router at 10.16.69.1 is
a normal eBGP speaker and the updates received by it from this peer will be just like a normal eBGP update from a peer in
autonomous system 6001.
In a BGP speaker in autonomous system 6002, the peers from autonomous systems 6001 and 6003 are configured as special eBGP
peers. 10.70.70.1 is a normal iBGP peer and 10.99.99.2 is a normal eBGP peer from autonomous system 700.
In a BGP speaker in autonomous system 6003, the peers from autonomous systems 6001 and 6002 are configured as special eBGP
peers. 10.200.200.200 is a normal eBGP peer from autonomous system 701.
The following is a part of the configuration from the BGP speaker 10.200.200.205 from autonomous system 701 in the same example.
Neighbor 172.16.232.56 is configured as a normal eBGP speaker from autonomous system 500. The internal division of the autonomous
system into multiple autonomous systems is not known to the peers external to the confederation.
Example: Route Reflector Using a Route Map to Set a Next Hop for an iBGP Peer
The following example is based on the figure above. Router 2 is the route reflector for the clients: Routers 1, 3, 4, and
5. Router 1 is connected to Router 3, but you don’t want Router 1 to forward traffic destined to AS 200 to use Router 3 as
the next hop (and therefore use the direct link with Router 3); you want to direct the traffic to the RR, which can load share
among Routers 3, 4, and 5.
This example configures the RR, Router 2. A route map named rr-out is applied to Router 1; the route map sets the next hop
to be the RR at 10.2.0.1. When Router 1 sees that the next hop is the RR address, Router 1 forwards the routes to the RR.
When the RR receives packets, it will automatically load share among the iBGP paths. A maximum of five iBGP paths are allowed.
The following example configures BGP dampening to be applied to prefixes filtered through the route map named ACCOUNTING:
ip prefix-list FINANCE permit 10.0.0.0/8
!
route-map ACCOUNTING
match ip address ip prefix-list FINANCE
set dampening 15 750 2000 60
exit
router bgp 50000
address-family ipv4
bgp dampening route-map ACCOUNTING
end
Example: Configuring BGP Route Map next-hop self
This section contains an example of how to configure BGP Route Map next-hop self.
In this example, a route map is configured that matches the networks where you wish to override settings for bgp next-hop
unchanged and bgp next-hop unchanged allpath. Subsequently, next-hop self is configured. After this, the bgp route map priority
is configured for the specified address family so that the previously specified route map takes priority over the settings
for bgp next-hop unchanged and bgp next-hop unchanged allpath. This configuration results in static routes being redistributed
with a next hop of self, but connected routes and routes learned via IBGP or EBGP continue to be redistributed with an unchanged
next hop.
route-map static-nexthop-rewrite permit 10
match source-protocol static
set ip next-hop self
route-map static-nexthop-rewrite permit 20
match route-type internal
match route-type external
match source-protocol connected
!
router bgp 65000
neighbor 172.16.232.50 remote-as 65001
address-family vpnv4
neighbor 172.16.232.50 activate
neighbor 172.16.232.50 next-hop unchanged allpaths
neighbor 172.16.232.50 route-map static-nexthop-rewrite out
exit-address-family
address-family ipv4 unicast vrf inside
bgp route-map priority
redistribute static
redistribute connected
exit-address-family
end
“Connecting to a Service Provider Using External BGP” module
Configuring features that apply to multiple IP routing protocols
IP Routing: Protocol-Independent Configuration Guide
RFCs
RFC
Title
RFC 1772
Application of the Border Gateway Protocol in the Internet
RFC 1773
Experience with the BGP Protocol
RFC 1774
BGP-4 Protocol Analysis
RFC 1930
Guidelines for Creation, Selection, and Registration of an Autonomous System (AS)
RFC 2519
A Framework for Inter-Domain Route Aggregation
RFC 2858
Multiprotocol Extensions for BGP-4
RFC 2918
Route Refresh Capability for BGP-4
RFC 3392
Capabilities Advertisement with BGP-4
RFC 4271
A Border Gateway Protocol 4 (BGP-4)
RFC 4893
BGP Support for Four-octet AS Number Space
RFC 5396
Textual Representation of Autonomous system (AS) Numbers
RFC 5398
Autonomous System (AS) Number Reservation for Documentation Use
Technical Assistance
Description
Link
The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use
these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products
and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.
Feature Information for Configuring Internal BGP Features
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature
Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for Configuring Internal BGP Features
Feature Name
Releases
Feature Configuration Information
Configuring internal BGP features
10.3
12.0(7)T
12.0(32)S12
12.2(33)SRA
12.2(33)SXH
All the features contained in this module are considered to be legacy features and will work in all trains release images.
The following commands were introduced or modified by these features:
bgpalways-compare-med
bgpbestpathmedconfed
bgpbestpathmedmissing-as-worst
bgpclient-to-clientreflection
bgpcluster-id
bgpconfederationidentifier
bgpconfederationpeers
bgpdampening
bgpdeterministicmed
clearipbgpdampening
clearipbgpflap-statistics
neighborroute-reflector-client
neighbortimers
showipbgp
showipbgpdampeningdampened-paths
showipbgpdampeningflap-statistics
timersbgp
BGP Outbound Route Map on Route Reflector to Set IP Next Hop
12.0(16)ST
12.0(22)S
12.2
12.2(14)S
15.0(1)S
The BGP Outbound Route Map on Route Reflector to Set IP Next Hop feature allows a route reflector to modify the next hop
attribute for a reflected route.
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