OSPF cost and route selection

[berniechen]

Hi

It is a quite simple question.  If there are multiple available paths from A to B (same area). Router will only update the shortest path(least cost) in the routing table or more than that?

Thanks in advance.

[Vivek Ganapathi]

yes you are right. Router will have the best path installed into its routing table (i.e the one with least cost). But it will keep the backup path always in its OSPF database. You can see that by issuing show ip ospf database command.

Hope it helps.

[Peter Paluch · Accepted solution]

Hello Xi,

I'd like to add to Vivek's answer that what OSPF really has in its database are only topological pieces of information from which the resulting shortest path can be computed - or "stitched together". OSPF does not really precompute the backup paths even if they are available, rather, it only maintains the data from which the backup path can be computed later. If the primary path fails, OSPF has to reprocess its link-state database to compute the new best path. It does not have the backup path readily available.

Best regards,

Peter

[Vivek Ganapathi]

Thanks Peter to complete the post. Yes, OSPF has to re-run the SPF algorithm to compute a backup path even though it has the topological roadmap of the entire network.

[viswamin]

Just to add..

the routing protocols will calculate the best path for all the network that it knows off and sends it to the router's processor.

The router now takes all these path, compare the AD of each of the network advertised by various routing protocols and then decide on what should get into the routing table. the cost comes in as a Tie Breaker

-Vijay

[Joseph W. Doherty]

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Posting

One additional note for the OP, OSPF can maintain equal cost routes in the route table.

[berniechen]

Thanks for all of you guys.