One of the routing protocols thats covered in IP6FD is Integrated ISIS. As the students work through the labs they come to a point where they add the following to the configuration:
interface Serial0/0/0.1 point-to-point isis ipv6 metric 21
This is supposed to change the metric and when they look at the output of the command
show isis ipv6 topology they expect the see a metric of 21. The problem is that you see the following:
R1#sh isis ipv6 top IS-IS IPv6 paths to level-1 routers System Id Metric Next-Hop Interface SNPA R1 -- R2 10 R2 Se0/0/0.1 DLCI 111 IS-IS IPv6 paths to level-2 routers System Id Metric Next-Hop Interface SNPA R1 --
So what’s the deal here? Well the answer is simple: Multi-topology is not enabled under the address-family for IPv6.
When you don’t enable multi-topology the router runs IS-IS IPv6 in single SPF mode and when you’re in single mode there is nowhere for ISIS to encode the IPv6 Metric. Multi-Topology allows the router to run ISIS for IPv6 independent of ISIS for IPv6, thus having two SPF process and now having a place to encode the IPv6 Metric.
To enable Multi-topology first enter the ISIS routing process:
Next enter the address family for ipv6:
And finally enable the Multi-topology feature.
Enabling or disabling multi-topology clears all adjacencies, all LSP databases, and all IS-IS ipv6 routes. When it all comes back you should now see the following result in the
show isis ipv6 topology command output:
R1#sh isis ipv6 topology IS-IS IPv6 paths to level-1 routers System Id Metric Next-Hop Interface SNPA R1 -- R2 21 R2 Se0/0/0.1 DLCI 111 IS-IS IPv6 paths to level-2 routers System Id Metric Next-Hop Interface SNPA R1 -- R1#
There we go! The metric now shows up properly configured.
Note: This configuration needs to be done on both sides of the adjacency.
Additionally you may be interested in enabling the
transition option which accepts and generates both IS-IS IPv6 and Multi-topology IPv6 TLVs. That configuration would resemble the following:
Integrated ISIS can be fun for IPv6 but there are the little nuances that you should beware of. You can easily lab this up in GNS3 or using the Cisco Learning Labs. Soon, when VIRL is released you’ll likely want to use it as your primary playground, testing the compatibility between different OS flavors. Regardless of how you lab it up, IPv6 is a protocol everyone should be getting familiar with.