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Example: Configuring IS-IS IPv4 and IPv6 Unicast Topologies

This example shows how to configure IS-IS to calculate an alternate IPv6 unicast topology, in addition to the normal IPv4 unicast topology.


No special configuration beyond device initialization is required before configuring this example.


This example focuses on IPv4 and IPv6 unicast topologies. The IS-IS interface metrics for the IPv4 topology can be configured independently of the IPv6 metrics. You can also selectively disable interfaces from participating in the IPv6 topology while continuing to participate in the IPv4 topology. This enables you to exercise control over the paths that unicast data takes through a network.

To enable an IPv6 unicast topology for IS-IS, include the ipv6-unicast statement:

To configure a metric for the IPv6 unicast topology, include the ipv6-unicast-metric statement:

To exclude an interface from the IPv6 unicast topologies for IS-IS, include the no-ipv6-unicast statement:

Figure 1 shows the topology used in this example. The black lines indicate link membership in the IPv6 topology. The gray lines indicate membership to the IPv4 topology. Using regular TLVs, it would not be possible to build multiple topologies and run an SPF calculation based on them. The multitopology extensions describe an extension to carry the set of supported protocols in the hello packet. After activating multitopology routing support on a link, the link carries all the topologies that the underlying circuit is able to relay.

Figure 1: IS-IS IPv4 and IPv6 Unicast TopologiesIS-IS IPv4 and IPv6 Unicast Topologies

CLI Quick Configuration shows the configuration for all of the devices in Figure 1. The section #configuration340__isis-ipv6-mtr-step-by-step describes the steps on Device R1.



CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device R1

Device R2

Device R3

Device R4

Device R5

Device R6

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure an alternate IPv6 unicast topology:

  1. Configure the interfaces.

  2. Enable IS-IS on the interfaces.

  3. Enable multitopology routing on the IS-IS interfaces.

    The ipv6-unicast statement enables multitopology IS-IS routing on all interfaces that have family iso and family inet6 configured and are listed at the [edit protocols isis interface] hierarchy level.

  4. Disable IPv6 unicast support on a given interface.

    If you do not want to run multitopology IS-IS routing for IPv6 on a given interface, you can disable multitopology routing by including the no-ipv6-unicast statement in the IS-IS interface configuration.


From configuration mode, confirm your configuration by entering the show interfaces and show protocols commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode.


Confirm that the configuration is working properly.

Checking the Topologies on Neighbors


Determine what topologies are supported on neighboring IS-IS devices.


From operational mode, enter the show isis adjacency detail command.


As expected, the adjacency with Device R5 only supports the IPv4 unicast topology, while the adjacencies with Device R2 and Device R3 support both the IPv4 and IPv6 topologies.

Checking the IS-IS SPF Calculations


Verify that separate SPF calculations are being run for IPv4 and IPv6.


From operational mode, enter the show isis spf brief command.


As expected, SPF calculations are being performed for IPv4 and IPv6 topologies.

Checking the Tcpdump Output


Verify that the link can be a member of both the IPv4 unicast topology and the IPv6 unicast topology.



The IS-IS hello (IIH) packet shows that IPv4 and IPv6 are supported. The hello packet lists valid IPv4 and IPv6 addresses, and therefore the routing device can create valid next-hop entries. The supported protocols are listed in the multitopology TLV #229.