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

Configure route customization for IS-IS

Updated: June 9, 2026

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Explains how to customize routes for IS-IS, including example procedures for redistributing IS-IS routes between multiple instances to support complex routing environments.

Enable advanced routing functions in IS-IS, including injecting default routes and redistributing routes between IS-IS instances.

Use this task to perform route customization in IS-IS. You can inject default routes, redistribute routes from other IS-IS instances, and adjust routing preferences. Customization is optional and depends on your network design.

Procedure

1.

Enable IS-IS routing for a specific instance.

Example:

Router(config)# router isis isp

Enables IS-IS routing for the specified routing process, and places the router in router configuration mode.

By default, all IS-IS instances are automatically Level 1 and Level 2. You can change the level of routing to be performed by a particular routing instance by using the is-type command.
2.

(Optional) Set the overload bit if needed.

Example:


Router(config-isis)# set-overload-bit
Note

Overload bit behavior does not apply during NSF restarts.

3.

Specify the address family.

Example:


Router(config-isis)# address-family ipv4 unicast
4.

(Optional) Inject a default route into the routing domain.

Example:


Router(config-isis-af)# default-information originate

  • If you specify a route policy, it controls when the route is advertised.

  • Without a route policy, the default route is advertised unconditionally at Level 2.
5.

(Optional) Redistribute routes from one IS-IS instance into another.

Example:


Router(config-isis-af)# redistribute isis 2 level-1

In this example, an IS-IS instance redistributes Level 1 routes from another IS-IS instance.
6.

(Optional) Summarize IPv4 or IPv6 prefixes.

Example:

Router(config-isis-af)# summary-prefix 10.1.0.0/16 level 1

or 


Router(config-isis-af)# summary-prefix 3003:xxxx::/24 level 1

  • IPv6 prefixes must be configured in IPv6 address family mode, IPv4 prefixes in IPv4 address family mode.
7.

(Optional) Configure the maximum number of parallel paths allowed.

Example:


Router(config-isis-af)# maximum-paths 16

Sets the maximum parallel paths in the routing table.
8.

(Optional) Define administrative distance for IS-IS routes. You can also configure the attached bit in Level 1 LSPs.

Example:


Router(config-isis-af)# distance 90

Example:


Router(config-isis-af)# set-attached-bit

Redistributing IS-IS routes between multiple instances

Redistributing IS-IS routes between multiple instances allows network administrators to propagate routing information across IS-IS instances configured with different levels (Level 1, Level 2). This process supports seamless area-to-backbone communication and ensures preferred route selection. Key commands include setting the attached bit and explicit redistribution.

Sample configuration

The example below shows two IS-IS instances:

  • Instance 1: Restricted to Level 2 (backbone)

  • Instance 2: Restricted to Level 1 (area)

Objective: The Level 1 instance propagates routes to the Level 2 instance using redistribution. The administrative distance is set higher for Level 2 so Level 1 routes are preferred. The attached bit is set for Level 1 to support backbone routing.


! Level 2 instance configuration
router isis 1
  is-type level-2-only
  net 49.0001.0001.0001.0001.00
  address-family ipv4 unicast
    distance 116
    redistribute isis 2 level 2
!
interface GigabitEthernet 0/3/0/0
  address-family ipv4 unicast
!
! Level 1 instance configuration
router isis 2
  is-type level-1
  net 49.0002.0001.0001.0002.00
  address-family ipv4 unicast
    set-attached-bit send always-set
!
interface GigabitEthernet 0/1/0/0
  address-family ipv4 unicast

  • set-attached-bit send always-set : Sets the attached bit for the Level 1 instance, allowing it to advertise backbone capability when redistributing routes.

  • redistribute isis 2 level 2 : Enables the propagation of routes from instance 2 (Level 1) to instance 1 (Level 2).

  • distance 116 : Sets the administrative distance higher for Level 2, making Level 1 routes preferred.