Interfaces can be IS-IS-enabled for IPv4, IPv6, or both. Within an IS-IS area, IPv4 and IPv6 can be supported in a single topology or in a multiple topology. A transition configuration is provide for migrating between the single and multiple topologies.
In a single topology, all interfaces in an area must be configured for IPv4, IPv6, or both IPv4 and IPv6. Adjacencies are denied if the topology configuration between two routers does not match. A single SPF calculation is performed for each route in a single topology area.
In a multiple topology area, each interface can be configured for IPv4, IPv6, or both IPv4 and IPv6. The router creates separate topologies for IPv4 and IPv6. Adjacencies are permitted between interfaces with different configurations, provided that the neighbors support at least one common protocol. Separate SPF calculations are computed for IPv4 and IPv6 routes.
Note
Although the IPv4 and IPv6 topologies can be different when multiple topologies are enabled, the topologies must be convex. That is, no IPv4 interface can be reachable only through traversal of IPv6 interfaces, and vice versa. All routers in an area must use the same topology mode.When the transition topology mode is configured, the router allows adjacencies between any two interfaces, and distributes both single topology and multiple topology TLVs. Transition mode permits both single and multiple topologies to coexist, but it generates more traffic. Transition mode should be used only for transitioning between modes and should be disabled after the transition is complete.
By default, a single topology is used for both IPv4 and IPv6. As a result, all interfaces must be homogeneously ISIS-enabled for IPv4, IPv6, or both.
To change the configured topology mode, see Configure the Multi-Topology Feature.
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