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The MPLS Traffic Engineering: Shared Risk Link Groups feature enhances backup tunnel path selection so that a backup tunnel avoids using links that are in the same Shared Risk Link Group (SRLG) as interfaces the backup tunnel is protecting.
SRLGs refer to situations where links in a network share a common fiber (or a common physical attribute). If one link fails, other links in the group may fail too. Links in the group have a shared risk.
You must configure Fast Reroutable tunnels.
You must enable the autotunnel backup.
The backup tunnel must be within a single area.
Manually created backup tunnels do not automatically avoid SRLGs of protected interfaces.
A primary tunnel cannot be specified to avoid links belonging to specified SRLGS.
Multiprotocol Label Switching (MPLS) is an Internet Engineering Task Force (IETF)-specified framework that provides for the efficient designation, routing, forwarding, and switching of traffic flows through the network.
Traffic engineering (TE) is the process of adjusting bandwidth allocations to ensure that enough is left for high-priority traffic.
In MPLS TE, the upstream router creates a network tunnel for a particular traffic stream, then fixes the bandwidth available for that tunnel.
SRLGs refer to situations where links in a network share a common fiber (or a common physical attribute). If one link fails, other links in the group may fail too. Links in the group have a shared risk.
Backup tunnels should avoid using links in the same SRLG as interfaces they are protecting. Otherwise, when the protected link fails the backup tunnel fails too.
The figure below shows a primary label-switched path (LSP) from router R1 to router R5. The LSP protects against the failure of the R2-R3 link at R2 via a backup tunnel to R4. If the R2-R3 link fails, link protection reroutes the LSP along the backup tunnel. However, the R2-R3 link and one of the backup tunnel links are in the same SRLG. So if the R2-R3 link fails, the backup tunnel may fail too.
The MPLS TE SRLG feature enhances backup tunnel path selection so a backup tunnel can avoid using links that are in the same SRLG as the interfaces it is protecting.
There are two ways for a backup tunnel to avoid the SRLGs of its protected interface:
The router does not create the backup tunnel unless it avoids SRLGs of the protected interface.
The router tries to avoid SRLGs of the protected interface, but if that is not possible the router creates the backup tunnel anyway. In this case there are two explicit paths. The first explicit path tries to avoid the SRLGs of the protected interface. If that does not work, the backup tunnel uses the second path (which ignores SRLGs).
 Note |
Only backup tunnels that routers create automatically (called autotunnel backup) can avoid SRLGs of protected interfaces. |
To activate the MPLS TE SRLG feature, you must do the following:
Configure the SRLG membership of each link that has a shared risk with another link.
Configure the routers to automatically create backup tunnels that avoid SRLGs of the protected interfaces.
Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS) flood the SRLG membership information (including other TE link attributes such as bandwidth availability and affinity) so that all routers in the network have the SRLG information for each link. With this topology information, routers can compute backup tunnel paths that exclude links having SRLGs in common with their protected interfaces. As shown in the figure below, the backup tunnel avoids the link between R2 and R3, which shares an SRLG with the protected interface.
Fast Reroute (FRR) protects MPLS TE LSPs from link and node failures by locally repairing the LSPs at the point of failure. This protection allows data to continue to flow on LSPs while their headend routers attempt to establish new end-to-end LSPs to replace them. FRR locally repairs the protected LSPs by rerouting them over backup tunnels that bypass failed links or nodes.
Backup tunnels that bypass only a single link of the LSP’s path provide link protection. They protect LSPs if a link along their path fails by rerouting the LSP’s traffic to the next hop (bypassing the failed link). These are referred to as next-hop (NHOP) backup tunnels because they terminate at the LSP’s next hop beyond the point of failure. The figure below illustrates an NHOP backup tunnel.
FRR provides node protection for LSPs. Backup tunnels that bypass next-hop nodes along LSP paths are called next-next-hop (NNHOP) backup tunnels because they terminate at the node following the next-hop node of the LSP paths, thereby bypassing the next-hop node. They protect LSPs if a node along their path fails by enabling the node upstream of the failure to reroute the LSPs and their traffic around the failed node to the next-next hop. FRR supports the use of Resource Reservation Protocol (RSVP) hellos to accelerate the detection of node failures. NNHOP backup tunnels also provide protection from link failures, because they bypass the failed link and the node.
The figure below illustrates an NNHOP backup tunnel.
Autotunnel backup is the ability of routers to create backup tunnels automatically. Therefore, you do not need to preconfigure each backup tunnel and then assign the backup tunnel to the protected interface. Only automatically created backup tunnels can avoid SRLGs or their protected interfaces.
For information about backup tunnels, see the Fast Reroute Protection for MPLS TE SRLGs.
For detailed information about autotunnel backup and how you can change the default command values, see MPLS Traffic Engineering (TE)--AutoTunnel Primary and Backup.
To globally activate the autotunnel backup feature, enter the mpls traffic-eng auto-tunnel backup command.
The figure below illustrates an NNHOP automatically generated backup tunnel that excludes the router 192.168.1.1 and terminates at router R4. The backup tunnel must avoid touching any links of 192.168.1.1.
The figure below illustrates an NHOP automatically generated backup tunnel that terminates at router R3 and avoids the link 10.1.1.1, not the entire node.
Note |
NNHOP excludes the router ID (the entire router must be excluded; that is, no link of the router can be included in the backup tunnel’s path). NHOP excludes only the link when the backup tunnel’s path is computed. |
Perform the following task to configure MPLS TE SRLG membership of each link that has a shared risk with another link. Configuring SRLG membership enhances backup tunnel path selection so that a backup tunnel avoids using links that are in the same SRLG as interfaces the backup tunnel is protecting.
Enter the commands on the physical interface.
| Command or Action | Purpose | |||
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Step 1 |
enable Example: |
Enables privileged EXEC mode.
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Step 2 |
configure terminal Example: |
Enters global configuration mode. |
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Step 3 |
interface type slot/port Example: |
Specifies an interface and enters interface configuration mode.
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Step 4 |
mpls traffic-eng srlg [number] [ Example: |
Configures the SRLG membership of a link (interface).
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Step 5 |
mpls traffic-eng srlg end Example: |
Exits to privileged EXEC mode. |
Perform the following task to configure routers that automatically create backup tunnels to avoid MPLS TE SRLGs of their protected interfaces. Backup tunnels provide link protection by rerouting traffic to the next hop bypassing failed links or in this instance by avoiding SRLGs.
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
enable Example: |
Enables privileged EXEC mode.
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Step 2 |
configure terminal Example: |
Enters global configuration mode. |
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Step 3 |
mpls traffic-eng auto-tunnel backup srlg exclude [force | preferred ] Example: |
Specifies that autocreated backup tunnels should avoid SRLGs of its protected interface.
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Step 4 |
end Example: |
Returns to privileged EXEC mode. |
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Step 1 |
enable Use this command to enable privileged EXEC mode. Enter your password, if prompted. For example: Example: |
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Step 2 |
show running-config Use the following commands to configure the SRLG membership of the interface pos 1/3/1 and to verify that the configuration is as expected. For example: Example:This verifies that the Packet over SONET (POS) interface pos 1/3/1 is associated that SRLG 1 and SRLG 2. |
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Step 3 |
show mpls traffic-eng link-management interfacesinterface slot/port Use this command to show the SRLG membership configured on interface pos 1/3/1. For example: Example: |
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Step 4 |
show mpls traffic-eng topology Use this command to show the SRLG link membership flooded via the Interior Gateway Protocol (IGP). For example: Example: |
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Step 5 |
show mpls traffic-eng topology srlg Use this command to display all the links in the network that are members of a given SRLG. For example: Example:The following command shows that there are two links in SRLG 1: Example: |
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Step 6 |
show mpls traffic-eng topology brief Use this command to display brief topology information: Example: |
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Step 7 |
show mpls traffic-eng link-management advertisements Use this command to show local link information that MPLS TE link management is currently flooding into the global TE topology. For example: Example: |
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Step 8 |
show ip rsvp fast-reroute Use this command to show that the primary tunnel is going over Pos1/3/1 on R3, on which SLRG 1 is configured. For example: Example: |
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Step 9 |
mpls traffic-eng auto-tunnel backup srlg exclude force Use the following commands to configure autotunnel backup with the force keyword. For example: Example: |
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Step 10 |
show ip explicit-paths Use the following command to verify that the force keyword is configured with the pos1/3/1 link excluded from the IP explicit path. For example: Example: |
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Step 11 |
show mpls traffic-eng tunnels tunnel num Use the following command to show that autotunnel backup is configured but is down because the headend router does not have any other path to signal and it cannot use pos1/2/1 because it belongs in the same SRLG; that is, SRLG 1. For example: Example: |
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Step 12 |
mpls traffic-eng auto-tunnel backup srlg exclude preferred The following commands configure autotunnel backup with the preferred keyword. For example: Example: |
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Step 13 |
show ip explicit-paths The following command shows two explicit paths. The first path avoids the SRLGs of the protected interface. The second path does not avoid the SRLGs. For example: Example: |
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Step 14 |
show ip rsvp fast-reroute The following command shows that the primary tunnel is protected with autotunnel backup using the second path option (see Step 10) that does not avoid the SRLGs. For example: Example:The following command shows the path options for the tunnel Tu65436: Example: |
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Step 15 |
exit Use this command to exit to user EXEC mode. For example: Example: |
The following example shows how to specify that the SRLG membership of each link has a shared risk with another link.
As shown in the figure below and in the following commands:
link R2-R3 = SRLG5
link R2-R3 = SRLG6
link R7-R4 = SRLG5
link R1-R2 = SRLG6
Router1# configure terminal
Router1# interface pos 1/0
Router1(config-if)# mpls traffic-eng srlg 6
Router2# configure terminal
Router2# interface pos 1/1
Router2(config-if)# mpls traffic-eng srlg 5
Router2(config-if)# mpls traffic-eng srlg 6
Router7# configure terminal
Router7# interface pos 3/0
Router7(config-if)# mpls traffic-eng srlg 5
The following example shows how to specify that automatically created backup tunnels are forced to avoid SRLGs of their protected interfaces:
Router# configure terminal
Router(config)# mpls traffic-eng auto-tunnel backup
Router(config)# mpls traffic-eng auto-tunnel backup srlg exclude force
The figure below illustrates the automatically created NNHOP backup tunnel that would be created to avoid SRLGs of the protected interface if the following conditions exist:
The exclude address is 192.168.1.1.
The link at R2 has an IP address of 10.1.1.1.
The backup tunnel’s explicit path avoids links that have a membership in the same SRLG as the link whose IP address is 10.1.1.1.
The figure below illustrates the automatically created NHOP backup tunnel that would be created.
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Related Topic |
Document Title |
|---|---|
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Fast Reroute |
MPLS TE: Link and Node Protection, with RSVP Hellos Support (with Fast Tunnel Interface Down Detection) |
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IS-IS |
Integrated IS-IS Routing Protocol Overview |
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OSPF |
Configuring OSPF |
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Autotunnel backups |
MPLS Traffic Engineering AutoTunnel Primary and Backup |
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Standard |
Title |
|---|---|
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None |
-- |
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MIB |
MIBs Link |
|---|---|
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None |
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: |
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RFC |
Title |
|---|---|
|
draft-ietf-isis-gmpls-extensions-16.txt |
IS-IS Extensions in Support of Generalized MPLS |
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Description |
Link |
|---|---|
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The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies. To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds. Access to most tools on the Cisco Support website requires a Cisco.com user ID and password. |
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 www.cisco.com/go/cfn. An account on Cisco.com is not required.|
Feature Name |
Releases |
Feature Information |
|---|---|---|
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MPLS Traffic Engineering: Shared Risk Link Groups |
12.0(28)S 12.0(29)S 12.2(33)SRA 12.2(33)SXH 12.4(20)T Cisco IOS XE Release 3.5S |
The MPLS Traffic Engineering: Shared Risk Link Groups feature enhances backup tunnel path selection so that a backup tunnel avoids using links that are in the same Shared Risk Link Group (SRLG) as interfaces the backup tunnel is protecting. SRLGs refer to situations where links in a network share a common fiber (or a common physical attribute). If one link fails, other links in the group may fail too. Links in the group have a shared risk. This document contains information about and instructions for configuring the MPLS Traffic Engineering Shared Risk Link Groups feature In 12.0(28)S, this feature was introduced. In 12.0(29)S, support was added for Open Shortest Path First (OSPF). In 12.2(33)SRA, this feature was integrated into a Cisco IOS 12.2SRA release In 12.2(33)SXH, this feature was integrated into a Cisco IOS 12.2SXH release. In 12.4(20)T, this feature was integrated into a Cisco IOS 12.4T release. In Cisco IOS XE Release 3.5S, this feature was integrated into Cisco IOS XE Release 3.5S. The following commands were introduced or modified: mpls traffic-eng auto-tunnel backup srlg exclude , mpls traffic-eng srlg , show ip explicit-paths , show mpls traffic-eng link-management advertisements , show mpls traffic-eng link-management interfaces , and show mpls traffic-eng topology . |
Fast Reroute --A mechanism for protecting MPLS traffic engineering (TE) LSPs from link and node failure by locally repairing the LSPs at the point of failure. This protection allows data to continue to flow on them while their headend routers attempt to establish end-to-end LSPs to replace them. FRR locally repairs the protected LSPs by rerouting them over backup tunnels that bypass failed links or nodes.
hop --Passage of a data packet between two network nodes (for example, between two routers).
IGP --Interior Gateway Protocol. An Internet protocol used to exchange routing information within an autonomous system.
interface --A network connection.
IP address --A 32-bit address assigned to hosts using TCP/IP. An IP address belongs to one of five classes (A, B, C, D, or E) and is written as four octets separated by periods (dotted decimal format). Each address consists of a network number, an optional subnetwork number, and a host number. The network and subnetwork numbers together are used for routing, and the host number is used to address an individual host within the network or subnetwork. A subnet mask is used to extract network and subnetwork information from the IP address.
IP explicit path --A list of IP addresses, each representing a node or link in the explicit path.
IS-IS --Intermediate System-to-Intermediate System. OSI link-state hierarchal routing protocol based on DECnet Phase V routing, where intermediate system (IS) routers exchange routing information based on a single metric to determine the network topology.
LDP --Label Distribution Protocol. A standard protocol between MPLS-enabled routers to negotiate the labels (addresses) used to forward packets.
link --A point-to-point connection between adjacent nodes.
LSP --label-switched path. A path that is followed by a labeled packet over several hops, starting at an ingress LSR and ending at an egress LSR.
LSR --label switching router. A Layer 3 router that forwards a packet based on the value of a label encapsulated in the packet.
MPLS --Multiprotocol Label Switching. A method for forwarding packets (frames) through a network. It enables routers at the edge of a network to apply labels to packets. ATM switches or existing routers in the network core can switch packets according to the labels with minimal lookup overhead.
node --An endpoint of a network connection or a junction common to two or more lines in a network. Nodes can be interconnected by links, and serve as control points in the network.
OSPF --Open Shortest Path First. A link-state hierarchical Interior Gateway Protocol (IGP) routing algorithm, derived from the IS-IS protocol. OSPF features include least-cost routing, multipath routing, and load balancing.
router --A network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.
router ID --Something by which a router originating a packet can be uniquely distinguished from all other routers; for example, an IP address from one of the router’s interfaces.
traffic engineering --The techniques and processes used to cause routed traffic to travel through the network on a path other than the one that would have been chosen if standard routing methods had been used.
tunnel --A secure communication path between two peers, such as two routers. A traffic engineering tunnel is a label-switched tunnel that is used for traffic engineering. Such a tunnel is set up through means other than normal Layer 3 routing; it is used to direct traffic over a path different from the one that Layer 3 routing could cause the tunnel to take.
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