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

Single-topology IPv6 for IS-IS

Updated: September 8, 2026

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Explains single-topology IPv6 operation in IS-IS and guides users through configuration steps to implement and manage single-topology IPv6 within network environments.

Single-topology IPv6 mode is an IS-IS routing approach that

  • enables IS-IS for IPv6 to be configured on interfaces alongside IPv4 network protocols

  • requires all interfaces and routers in an IS-IS area or domain to support identical network layer protocols, and

  • uses a shared link topology allowing a single shortest path first (SPF) computation for both IPv4 and IPv6 routes.

In single-topology mode, IPv6 topologies work with both narrow and wide metric styles, as used in IPv4 unicast topology. During single-topology operation, one shortest path first (SPF) computation for each level is used to compute both IPv4 and IPv6 routes. This is possible because both IPv4 IS-IS and IPv6 IS-IS routing protocols share a common link topology.

Configure single topology mode for IS-IS

Enable IS-IS single topology mode so IPv6 uses the existing IPv4 topology.

Use single topology mode when you want IPv6 to follow the IPv4 topology instead of using a separate topology. This simplifies routing configuration when both IP families share the same network design.

Before you begin

  • Confirm all required address families are enabled on IS-IS interfaces.

  • The single-topology command must be entered within the IPv6 address-family submode.

  • If you use adjacency-check disable , you do not need to enable all address families per interface.

  • The default metric style is narrow; configure metrics in the IPv4 or IPv6 address-family stanza as needed.

Procedure

  1. Configure the interface and IP addresses.

    Example:

    Router# configure
Router(config)# router isis isp
Router(config-isis)# interface HundredGigE 0/1/0/3
    Use one these commands:

    • For IPv4:

      ipv4 address address mask

    • For IPv6:

      ipv6 address ipv6-prefix / prefix-length> eui-64

      ipv6 address ipv6-address link-local

      ipv6 enable

      Notes:

      • If IS-IS uses IPv4, configure IPv4 addressing on all required interfaces.

      • Use the eui-64 keyword to derive the IPv6 address from the interface ID.

      • Use the link-local keyword for a specific link-local address.

  2. Configure IS-IS routing and enable single topology mode.

    Example:

    Router# configure
Router(config)# router isis isp
Router(config-isis)# net 47.0004.004d.0001.0001.0c11.1110.00
Router(config-isis)# address-family ipv6 unicast
Router(config-isis-af)# single-topology
Router(config-isis-af)# exit

    • Add multiple NETs if advertising more than one area address.

    • Use the same system ID in all NETs for the same instance.

  3. Configure IS-IS interface parameters and address families.

    Example:

    
Router(config-isis)# interface HundredGigE 0/1/0/3
Router(config-isis-if)# circuit-type level-1-2
Router(config-isis-if)# address-family ipv4 unicast
Router(config-isis-if)# commit

  4. (Optional) Verify the IS-IS interface state.

    Example:

    Router# show isis interface HundredGigE 0/1/0/3

  5. Verify the IS-IS topology view.

    Example:

    Router# show isis topology

Single topology IS-IS for IPv4 and IPv6

router isis isp
 net 49.0000.0000.0001.00
 address-family ipv6 unicast
  single-topology
 interface POS0/3/0/0
  address-family ipv4 unicast
  address-family ipv6 unicast
!
interface POS0/3/0/0
 ipv4 address 10.0.1.3 255.255.255.0
 ipv6 address 2001::1/64