IS-IS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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IS-IS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

Global weighted SRLG protections

Updated: June 9, 2026

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Explains global weighted SRLG protection, details its impact on backup path selection and network resiliency with IS-IS, and outlines prerequisites, configuration types, and distribution methods for SRLG data.

A global weighted SRLG protection is a path selection mechanism that

  • associates a weight with the SRLG value

  • uses the SRLG weights to compute backup paths, and

  • improves network resiliency by optimizing backup path selection in IS-IS routing.

A shared risk link group (SRLG) is a set of links that share a common resource and thus the same risk of failure. Existing loop-free alternate (LFA) implementations in interior gateway protocols (IGPs) support SRLG protection by considering only directly connected links for backup paths. This approach may fail if non-directly connected links sharing the same SRLG are part of the backup path calculation.

Additional details

To support global weighted SRLG protections, you need information about SRLGs on all links within the area topology. SRLG data for remote links can be distributed through IS-IS flooding or by manually configuring remote SRLGs.

Configure global weighted SRLG protection

Enable global weighted SRLG protection in your network to enhance failure recovery by considering SRLG weights during the computation of backup paths in IGPs.

Use global weighted SRLG protection to enhance network resilience by assigning weights to SRLGs during backup-path computation. Depending on your network design, you can configure:

  • Local SRLG values

  • Remote SRLG flooding (to advertise values dynamically), or

  • Static provisioning of remote SRLG values (when flooding is not possible).

Before you begin

Determine whether you want to use local SRLG protection, remote SRLG flooding, or static provisioning for remote links.

Follow these steps to configure global weighted SRLG protection (choose the workflow that matches your network requirements):

Procedure

1.

Configure local SRLG values and enable global weighted SRLG protection on the local router.

Example:

Router(config)# srlg
Router(config-srlg)# interface TenGigE0/0/0/0
Router(config-srlg-if)# name group1
Router(config-srlg-if)# exit
Router(config-srlg)# interface TenGigE0/0/0/1
Router(config-srlg-if)# name group1
Router(config-srlg)# name group value 100
Router(config)# router isis 1
Router(config-isis)# address-family ipv4 unicast
Router(config-isis-if-af)# fast-reroute per-prefix srlg-protection weighted-global
Router(config-isis-if-af)# fast-reroute per-prefix tiebreaker srlg-disjoint index 1
Router(config-isis)# interface TenGigE0/0/0/0
Router(config-isis-if)# point-to-point
Router(config-isis-if)# address-family ipv4 unicast
Router(config-isis-if-af)# fast-reroute per-prefix
Router(config-isis-if-af)# fast-reroute per-prefix ti-lfa
Router(config-isis)# srlg
Router(config-isis-srlg)# name group1
Router(config-isis-srlg-name)# admin-weight 5000
2.

Configure remote SRLG flooding on the local router.

Example:

Router(config)# router isis 1 
Router(config-isis)# address-family ipv4 unicast
Router(config-isis-if-af)# fast-reroute per-prefix srlg-protection weighted-global
Router(config-isis-if-af)# fast-reroute per-prefix tiebreaker srlg-disjoint index 1
Router(config-isis-if-af)# exit
Router(config-isis)# interface TenGigE0/0/0/0
Router(config-isis-if)# point-to-point 
Router(config-isis-if)# address-family ipv4 unicast
Router(config-isis-if-af)# fast-reroute per-prefix 
Router(config-isis-if-af)# fast-reroute per-prefix ti-lfa
Router(config-isis-if-af)# exit
Router(config-isis)# srlg
Router(config-isis-srlg)# name group1 
Router(config-isis-srlg-name)# admin-weight 5000
3.

Configure remote SRLG flooding when you want remote links to advertise their SRLG values dynamically.

Example:

Router(config)# srlg
Router(config-srlg)# interface TenGigE0/0/0/0
Router(config-srlg-if)# name group1
Router(config-srlg-if)# exit
Router(config-srlg)# interface TenGigE0/0/0/1
Router(config-srlg-if)# name group1
Router(config-srlg)# name group value 100
Router(config-srlg)# exit
Router(config)# router isis 1
Router(config-isis)# address-family ipv4 unicast
Router(config-isis-af)# advertise application lfa link-attributes srlg
4.

Configure static provisioning for remote SRLG values when flooding is not used. Perform these configurations on the local router.

Example:

Router(config)# srlg
Router(config-srlg)# interface TenGigE0/0/0/0
Router(config-srlg-if)# name group1
Router(config-srlg-if)# exit
Router(config-srlg)# interface TenGigE0/0/0/1
Router(config-srlg-if)# name group1
Router(config-srlg)# name group value 100
Router(config-srlg)# exit
Router(config)# router isis 1
Router(config-isis)# address-family ipv4 unicast
Router(config-isis-if-af)# fast-reroute per-prefix srlg-protection weighted-global
Router(config-isis-if-af)# fast-reroute per-prefix tiebreaker srlg-disjoint index 1
Router(config-isis)# interface TenGigE0/0/0/0
Router(config-isis-if)# point-to-point
Router(config-isis-if)# address-family ipv4 unicast
Router(config-isis-if-af)# fast-reroute per-prefix
Router(config-isis-if-af)# fast-reroute per-prefix ti-lfa
Router(config-isis)# srlg
Router(config-isis-srlg)# name group1
Router(config-isis-srlg-name)# admin-weight 5000
Router(config-isis-srlg-name)# static ipv4 address 10.0.4.1 next-hop ipv4 address 10.0.4.2
Router(config-isis-srlg-name)# static ipv4 address 10.0.4.2 next-hop ipv4 address 10.0.4.1