Example
Legend:
- + = Router
- - or | = Link
- (X) = Metric of link
Now a client on router E wants to talk to a client on router A. That means a route between router A and router E must be available. This route is calculated as follows:
The immediate neighbours of router E are router C and router D. DUAL in router E asks for the reported distance from routers C and D respectively to router A. The following are the results:
- Destination: Router A
- via D: RD(4)
- via C: RD(3)
The route via C is therefore in the lowest cost. In the next step, the distance from router E to the neighbours are added to the reported distance to get the feasible distance (FD):
- Destination: Router A
- via D: RD(4), FD(5)
- via C: RD(3), FD(6)
DUAL therefore finds that the route via D has the least total cost. Then the route via D will be marked as "successor", equipped with passive status and registered in the routing table. The route via C is kept as a "feasible successor", because its RD is less than the FD of the successor:
- Destination: Router A
- via D: RD(4), FD(5) successor
- via C: RD(3), FD(6) feasible successor
Read more about this topic: Diffusing Update Algorithm
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