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

PDF

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

IP fast reroute

Updated: June 9, 2026

Want to summarize with AI?

Log in

Introduces the principles and mechanisms of IP fast reroute, outlining how rapid failover techniques preserve network connectivity and minimize traffic disruption during link or node failures.

A fast reroute mechanism is a network resiliency technique that

  • computes alternate repair paths locally to protect against link failures

  • prevents packet loss caused by loops during network reconvergence, and

  • applies to IP routing networks without requiring MPLS.

Table 1. Feature History Table

Feature Name

Release Name

Description

IP Fast Reroute

Release 25.4.1

Introduced in this release on: Fixed Systems (8700 [ASIC: K100])(select variants only*)

*This feature is supported on Cisco 8711-48Z-M routers.

IP Fast Reroute

 Release 25.1.1

Introduced in this release on: Fixed Systems (8010 [ASIC: A100])(select variants only*)

*This feature is supported on Cisco 8011-4G24Y4H-I routers.

IP Fast Reroute

Release 24.4.1

Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100, K100])(select variants only); Modular Systems (8800 [LC ASIC: P100])(select variants only*)

This feature protects against link failure by using locally computed repair paths, preventing packet loss from loops during network reconvergence.

*This feature is supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 8712-MOD-M

  • 88-LC1-36EH

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

IP fast reroute (IPFRR) uses loop-free alternate (LFA) computation to protect against link failure. It employs locally computed repair paths to prevent packet loss caused by loops during network reconvergence after a failure. Unlike Multiprotocol Label Switching (MPLS), IPFRR is applicable to networks using conventional IP routing and forwarding.

For further technical details, refer to:

  • IETF draft-ietf-rtgwg-ipfrr-framework-06.txt

  • IETF draft-ietf-rtgwg-lf-conv-frmwk-00.txt

For guidance on configuring MPLS IPFRR, see the MPLS Configuration Guide.

Configure IP or LDP fast reroute loop-free alternate for an IS-IS interface

Improve network resiliency by enabling IP or LDP fast reroute loop-free alternate so your router automatically redirects traffic after a link or node failure.

Use this task on Cisco IOS XR devices to configure IP/LDP fast reroute loop-free alternate (LFA) for IS-IS interfaces. IPFRR and MPLS FRR must not be enabled on the same interface at the same time. For node protection on broadcast links, enable both IPFRR (fast reroute) and BFD under IS-IS for the interface.

Before you begin

  • Confirm your device runs Cisco IOS XR and supports IPFRR for IPv4 single-level interfaces.

  • Do not configure MPLS FRR and IPFRR at the same time on an interface.

Procedure

1.

Enable IS-IS routing and select the target interface.

Example:

Router(config)# router isis isp
RouterRouter(config-isis)# interface GigabitEthernet 0/1/0/3
2.

(Optional) Set the IS-IS circuit type (adjacency level).

Example:

RouterRouter(config-isis-if)# circuit-type level-1
3.

Enter address family configuration mode for IPv4 unicast and specify the fast reroute computation method and level.

Example:

Router(config-isis-if)# address-family ipv4 unicast
Router(config-isis-if-af)# fast-reroute per-link level 1
Router(config-isis-if-af)# fast-reroute per-prefix level 2

per-link: Prefix-independent; per-prefix: Prefix-dependent.
4.

(Optional) Exclude interfaces from FRR computation or define LFA candidates.

Example:

Router(config-isis-if-af)# fast-reroute per-link exclude interface Loopback0 level 1
Router(config-isis-if-af)# fast-reroute per-prefix lfa-candidate interface MgmtEth0/RP0/CPU0/0 level 1
5.

Enable IPFRR LFA for the chosen level and (optionally) exclude further interfaces.

Example:

Router(config-isis-if-af)# ipfrr lfa level 1
Router(config-isis-if-af)# ipfrr lfa exclude interface POS0/1/0/4