IPv6 Single Topology
Load isis.ipv6.init.cfg
All routers have IPv6 link-local addresses enabled on their interfaces, and a loopback IPv6 address of 2001:db8::<router num>/128.
Without enabling wide metrics, configure IPv6 reachability between all routers on their loopbacks.
Answer
Explanation
This is a disruptive change, because the network layer protocol ID field must match between routers in the ISIS Hello. When you enable IPv6 for ISIS on an interface, the router starts generating an ISIS Hello with two NLPIDs:
The other router, which still has only IPv4 at this point, will reject the adjacency now.
Once all routers have IPv6 for ISIS enabled on all interfaces, you should have end-to-end IPv6 reachability.
IOS-XE’s default IPv6 mode is single topology. In single topology, IPv6 “uses” the IPv4 topology. The IPv6 prefixes ride on top of the IPv4 topology and its adjacencies. IPv6 prefixes are simply advertised with TLV 236. This is used whether or not wide metrics are used. (Either can be used with single topology).
The link-local address, used for the nexthop by the neighbor for IPv6 routes, is advertised in the Hello in the IPv6 interface address TLV (232).
On IOS-XR, we must enable single-topology under the AFI. By default IOS-XR uses multi topology, in which IPv4 and IPv6 each have a separate topology, and IPv6 is not required to be enabled on every interface.
In summary:
IPv6 can be used with either single topology or multi topology
In single topology, IPv6 uses the IPv4 topology. IPv6 prefixes are advertised in a separate IPv6 reachability TLV (236)
Single topology is the default mode on IOS-XE
Multi topology is the default mode on IOS-XR
In single topology, every link must have both IPv4 and IPv6. If any router does not have IPv6 enabled, both IPv4 and IPv6 reachability is broken.
This is because the adjacency requires all protocols be supported between both neighbors
The exception is that you can use single topology and only IPv6. No IPv4 anywhere in the topology. In this case, the protocols supported TLV lists only IPv6 by itself.
Single topology can use either narrow or wide metrics. Multi topology requires wide metrics.
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