Wednesday, November 11, 2015

OSPF Flood War Caused by Duplicate Router-IDs

                                                (click on the images to view in full size)

If you see this error on a router it means you have 2 or more routers in the same area which has same OSPF router-id configured. Situation is called OSPF flood war.
Router-ID is a unique adjacency attribute which the SPF algorithm use as nodes to solve shortest paths to routes, in the link state graph.

Flood war occours when someone is advertising a route into the network & someone is withdrawing it.

Here a duplicate router advertises networks by LSAs which the other router thinks "old information" which needs to get aged out of it's database.

So the 2nd router reflood the LSA with the maximum age telling everyone to delete it.

side note: This mechanism is there in OSPF because if a router restarts and a connected link of that router is down during the reboot process, and if it received it's old router LSA which advertises it's old link which should not be circulating anymore. So the router floods the LSA with the maximum age telling others to ignore it.

So to troubleshoot this you must log into all routers in same area to see which router shows the same output & change one to a different ID.

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