All routers in the area or domain must use the same type of IPv6 support, either single topology or multi topology. A router operating in multi topology mode will not recognize the ability of the single topology mode router to support IPv6 traffic, which will lead to holes in the IPv6 topology. To transition from single topology to a more flexible multi topology support, a multi topology transition mode is provided.

The multi topology transition mode allows a network operating in single topology IS-IS IPv6 support mode to continue to work while upgrading routers to include multi topology IS-IS IPv6 support. While in transition mode, both types of TLVs (single-topology and multi topology) are sent in LSPs for all configured IPv6 addresses, but the router continues to operate in single topology mode (that is, the topological restrictions of the single topology mode are still in effect). After all routers in the area or domain have been upgraded to support multi topology IPv6 and are operating in transition mode, transition mode can be removed from the configuration. Once all routers in the area or domain are operating in multi topology IPv6 mode, the topological restrictions of single topology mode are no longer in effect.