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Subscribers with Identical Framed Routes

Subscribers in the same routing instance are typically expected to have different framed routes. However, there is an active/backup use case where you might want to configure the same framed route for two subscribers. In this scenario, a subscriber expects to receive ingress traffic from an active CPE device, but wants to switch as soon as possible to a backup CPE device.

You can meet this requirement by having two subscribers connected to the same BNG with the same access route for an identical Framed-Route address. However, you must configure a distance value in the Framed-Route that is different between the two subscribers. For example, you might configure RADIUS as follows:

Subscribers user1 and user2 have the same password, routing instance, and Framed-Route address. The distance is just an administrative distance or preference for discrimination between the routes. The distance is 12 for user1 and 240 for user2. The router can add only one route to the forwarding table. It selects the route with the lowest distance value, which is 12. Consequently, traffic towards the subscriber travels to the logical interface associated with user1.

The router installs the backup route for user2 in the routing table. If the link to user1 goes down, then the router installs the backup route for user2 in the forwarding table so downstream traffic can continue to the subscriber.

What happens if you do not configure different distance values for the two subscribers? Consider the following RADIUS configuration:

If both of these subscribers try to log in, only the one that logs in first achieves the Active state, so only that route is installed in the forwarding table. The other subscriber flaps between the Init and Terminated states and never succeeds at logging in, as long as the first subscriber is Active.