27.1.5 Introduction of external AS routes

The following figure shows an example of injecting external AS routes in a configuration that includes a backup line.

Figure 27-4: Example of introducing external AS routes in a configuration using a backup-line

In OSPFv3, packets are exchanged at regular intervals to detect neighboring routers. If a backup line is incorporated into the OSPFv3 topology, the backup line stays active because it is called on consistently to carry these packets. However, if you want the backup line to enter an idle state when it is not required for communication, perform the configuration described below.

On Switch A, enable OSPFv3 on the primary line. On the backup line, configure a static route to Network A. As the distance of the static route, assign a larger value (lower priority) than the distance for an internal OSPFv3 route. This results in the internal AS route learned by the OSPFv3 protocol being selected as the route to Network A. In the event of a primary line failure, the relevant internal AS route is deleted from Switch A, and the static route is re-selected. However, the routing information to Network A will no longer exist in Switch C. To advertise an external AS route to Switch C that describes the static route to Network A, you need to configure route redistribution at Switch A. By doing so, information about a valid route to Network A can be injected into OSPFv3 should the primary line fail, without hello packets being exchanged over the backup line.