Connected: An Internet Encyclopedia
16.4. Calculating AS external routes

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### 16.4. Calculating AS external routes

16.4. Calculating AS external routes

Call the routing table distance to the forwarding address X (when the forwarding address is set to 0.0.0.0, this is the distance to the ASBR itself), and the cost specified in the advertisement Y. X is in terms of the link state metric, and Y is a type 1 or 2 external metric.

4. Next, look up the routing table entry for the destination N. If no entry exists for N, install the AS external path to N, with next hop equal to the list of next hops to the forwarding address, and advertising router equal to ASBR. If the external metric type is 1, then the path-type is set to type 1 external and the cost is equal to X+Y. If the external metric type is 2, the path-type is set to type 2 external, the link state component of the route's cost is X, and the type 2 cost is Y.

5. Else, if the paths present in the table are not type 1 or type 2 external paths, do nothing (AS external paths have the lowest priority).

6. Otherwise, compare the cost of this new AS external path to the ones present in the table. Type 1 external paths are always shorter than type 2 external paths. Type 1 external paths are compared by looking at the sum of the distance to the forwarding address and the advertised type 1 metric (X+Y). Type 2 external paths are compared by looking at the advertised type 2 metrics, and then if necessary, the distance to the forwarding addresses.

If the new path is shorter, it replaces the present paths in the routing table entry. If the new path is the same cost, it is added to the routing table entry's list of paths.