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OSPF Adjcencies limit?


Q. Is there a definitive OSPF adjcency limit you should work to for a router?

I ask after seeing a core network with 6509-NEB-A's chassis seeing CPU spikes for long periods (causing network melt down!) due to excessive OSPF algorithm. Adjacencies reached several 100 per node. Passive-interface config was used in this instance to remove duplicate adjcencies between SVIs that were not required.

But if these are genuinely needed (such as many GRE tunnel /30s or high capacity line cards with many /30 connections being terminated), passive-interfaces would not be an option.

Is there a better solution to having adjacencies (without static routes everywhere), or is this just a limit to the routing protocol?

Any advice welcomed.


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Re: OSPF Adjcencies limit?


I do not believe that there is a definitive limit on the number of OSPF adjacencies per router. As you have discovered it is certainly possible to overload a router by creating too many adjacencies. But the limit will depend on multiple factors including what router plarform, how much memory that router is provisioned with, what other features are enabled on the router, what is the size of the routing table, and what is the size of the OSPF data bases.  It may also depend on how OSPF is configured:

- the worst case is to have many neighbors all in the same area.

- slightly better is to have many neighbors with neighbors belonging to multiple areas on the router.

- even better is to have many neighbors with neighbors belonging to multiple areas and to have all non-backbone areas configured as totally stubby.

So are there limits - yes there certainly are.

Are there definitive limits, no there are not.