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

Conditional default route originations in IS-IS

Updated: September 8, 2026

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Details conditional default route origination in IS-IS influenced by BGP neighbor status and provides instructions for configuring this functionality to enhance routing flexibility.

Conditional default route origination is an IS-IS routing feature that

  • enables IS-IS to originate a default route only when specific BGP neighbor or route conditions are satisfied,

  • improves network reliability and responsiveness by linking the advertisement of default routes to dynamic routing states, and

  • prevents incorrect default advertisements when BGP neighbors or watched routes are not active.

Table 1. Feature History Table

Feature Name

Release Information

Feature Description

Conditional Default Route Originating in IS-IS

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.

Conditional Default Route Originating in IS-IS

 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.

Conditional Default Route Originating in IS-IS

 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 is supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 8712-MOD-M

  • 88-LC1-36EH

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

Conditional Default Route Originating in IS-IS

 Release 7.3.2

The Conditional Default Route Originating in IS-IS feature allows you to enhance the granularity of the default route the IS-IS originates based on a condition. It enables IS-IS to originate the default route based on the presence of a specific route in the RIB originated by a particular BGP speaker.

This feature improves the reaction time of the watched route in the RIB by avoiding periodical queries of the routing policy. This feature enables you to respond to the client in a timely fashion when the watched route changes in the RIB.

Conditional default route origination in IS-IS enables administrators to control whether IS-IS advertises a default route into the network based on real-time conditions such as the state of a specific BGP neighbor or the presence of certain BGP-learned routes in the Routing Information Base (RIB). This dynamic behavior ensures the default route is only distributed when upstream connectivity is truly available.

Configure conditional default route origination in IS-IS based on BGP neighbor status

Control IS-IS default route advertisement based on BGP neighbor status or watched-route state.
Use this configuration to prevent advertising a default route unless specific BGP neighbor or route conditions are met, enhancing routing decision accuracy and stability.

Before you begin

  • IS-IS must be configured on the router.

  • BGP neighbor tracking should be defined.

  • Required route-policy design must be in place.

Procedure

  1. Configure the IS-IS instance and apply the route-policy under the IPv4 and IPv6 address families.

    Example:

    Router(config)#router isis 1
Router(config-isis)#is-type level-2-only
Router(config-isis)#net 47.0000.0000.0005.00
Router(config-isis)#address-family ipv4 unicast
Router(config-isis-af)#metric-style wide
Router(config-isis-af)#mpls traffic-eng level-2-only
Router(config-isis-af)#mpls traffic-eng router-id 10.5.5.5
Router(config-isis-af)#default-information originate route-policy policy_1
Router(config-isis-af)#segment-routing mpls sr-prefer
Router(config-isis-af)#exit
Router(config-isis)#address-family ipv6 unicast
Router(config-isis-af)#metric-style wide
Router(config-isis-af)#default-information originate route-policy policy_1
Router(config-isis-af)#segment-routing mpls sr-prefer
Router(config-isis-af)#exit
Router(config-isis)#exit

  2. Configure a route policy that originates the default route based on the BGP neighbor tracking state.

    Example:

    Router(config)#route-policy track_bgp_neighbor
Router(config-rpl)#if track track-bgp-neighbors is up then
Router(config-rpl-if)#pass
Router(config-rpl-if)#endif
Router(config-rpl)#end-policy

  3. Configure the watched-route policy, neighbor tracking, and IPv6 prefix-set objects used by the route policy.

    Example:

    Router(config)#route-policy track-bgp-neighbors
Router(config-rpl)#if rib-has-route async (192.1.1.0/24, 192.1.2.0/24) and source in (10.2.35.1) and track track-bgp-neighbors is up then
Router(config-rpl-if)#pass
Router(config-rpl-if)#endif
Router(config-rpl)#end-policy
Router(config)#track track-bgp-neighbors
Router(config-track)#type bgp neighbor address-family state
Router(config-track)#exit
Router(config)#address-family ipv4 unicast
Router(config)#neighbor 10.2.35.1
Router(config)#prefix-set bgp_ipv6_neighbor_id
Router(config-pfx)#10:2:35::1
Router(config-pfx)#end-set
Router(config)#prefix-set bgp_ipv6_watched_routes
Router(config-pfx)#192:1:1::/112
Router(config-pfx)#192:1:2::/112
Router(config-pfx)#end-set
Router(config)#route-policy default_route_policy_ipv6
Router(config-rpl)#if rib-has-route async bgp_ipv6_watched_routes and protocol is bgp 100 and source in bgp_ipv6_neighbor_id then
Router(config-rpl-if)#pass
Router(config-rpl-if)#else
Router(config-rpl-if)#drop
Router(config-rpl-if)#endif
Router(config-rpl)#end-policy

  4. Verify the BGP neighbor state, watched routes, tracking state, and the default route advertisement.

    Example:

    Verify the BGP neighbor status. 
    
Router(config)# show bgp neighbor brief 
Mon Mar 8 13:30:27.312 IST
Neighbor    Spk AS Description      Up/Down NBRState
10.2.35.1    0  100 02:18:39         Established 
10:2:35::1   0  100 02:18:40            Established

    Example:

    Verify the IPv4 RIB route. 
    
Router# show route ipv4 192.1.1.0/24 
Mon Mar 8 13:33:14.726 IST
Routing entry for 192.1.1.0/24 
   Known via "bgp 100", distance 200, metric 0, type internal 
   Installed Mar 8 11:11:52.738 for 02:21:22 
   Routing Descriptor Blocks 
     10.2.35.1, from 10.2.35.1 
       Route metric is 0 
  No advertising protos.

    Example:

    Verify the IPv6 RIB route. 
    Router# show route ipv6 192:1:1::/112 
Mon Mar 8 13:33:31.340 IST
Routing entry for 192:1:1::/112 
    Known via "bgp 100", distance 200, metric 0, type internal 
    Installed Mar 8 11:11:52.738 for 02:21:38 
    Routing Descriptor Blocks 
      10:2:35::1, from 10:2:35::1 
         Route metric is 0 
      No advertising protos.

    Example:

    Verify BGP neighbor status tracking. 
    Router# show track track-bgp-neighbors
Mon Mar 8 13:52:16.746 IST
Track track-bgp-neighbors 
    BGP Neighbor AF IPv4 Unicast NBR 10.2.35.1 vrf default 
    Reachability is UP 
       Neighbor Address Reachablity is Up 
       BGP Neighbor Address-family state is Up 
    12 changes, last change IST Mon Mar 08 2021 11:11:52.741 
    Delay up 0 secs(default), down 0 secs(default)

    Example:

    Verify the default route status in IS-IS address family. 
    Router# show isis 
Mon Mar 8 13:34:39.412 
IST IS-IS Router: 1 
    System Id: 0000.0000.0005 
    Instance Id: 0 
    IS Levels: level-2-only 
    Manual area address(es): 47 
    Routing for area address(es): 47
!! . . 
Topologies supported by IS-IS: 
   IPv4 Unicast 
    .  
    .
      Originating default route active since Mar 08 2021 11:12:05.914 IST 
   IPv6 Unicast 
   . 
   . 
      Originating default route active since Mar 08 2021 11:12:05.917 IST
!!

    Example:

    Verify the IS-IS database. 
    
Router# show isis database detail verbose r5 | i 0.0.0.0/0
Mon Mar 8 13:47:10.624 IST Metric: 0 IP-Extended 0.0.0.0/0 

Router# show isis database detail verbose r5 | i ::/0
Mon Mar 8 13:47:10.727 IST Metric: 0 MT (IPv6 Unicast) IPv6 ::/0

    Example:

    Verify the IPv4 IS-IS routes. 
    
Router# show isis ipv4 route 0.0.0.0/0 
Mon Mar 8 13:44:58.226 IST 
L2 0.0.0.0/0 [10/115] 
    via 10.1.35.2, TenGigE0/0/0/31, r5, SRGB Base: 16000, Weight: 0

    Example:

    Verify the IPv6 IS-IS routes. 
    Router# show isis ipv6 route 0::0/0 
Mon Mar 8 13:45:02.699 IST 
L2 ::/0 [10/115] 
  via fe80::28a:96ff:fee7:f418, TenGigE0/0/0/31, r5, SRGB Base: 16000, Weight: 0