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OSPF question about intermittent line interruption

Just for my interest for the OSPF operation. Anyone knows that if OSPF network formed by several router. What is the service impact if one of the router is being disconnected within Area 0, that means other OSPF neighbors will have the routing interruption or networks shaking within Area 0?

Second, if the router has intermittent line interruption in Area 0, it would make the OSPF topology change frequently so that the network will become unstable and the CPU of the router will be maintained in high usage level?

Thank you!

4 REPLIES
Cisco Employee

OSPF question about intermittent line interruption

Hello Kin,

What is the service impact if one of the router is being disconnected  within Area 0, that means other OSPF neighbors will have the routing  interruption or networks shaking within Area 0?

All routers within the area, and perhaps even outside the area, will be recalculating new shortest paths. However, the recalculation is usually done in such a way that the routing table contents are not changed during the SPF computation, and only the changed routes are modified in the routing table afterwards. This means that paths not affected by the disconnected router should not experience any outages.

Second, if the router has intermittent line interruption in Area 0, it  would make the OSPF topology change frequently so that the network will  become unstable and the CPU of the router will be maintained in high  usage level?

This is true - the intermittent connection would cause constant topology changes to which OSPF is required to respond. It could indeed keep CPUs on routers in the area busier than usual. The instability of the network is primarily concerned with all the paths that pass through the router with the intermittent connection. For paths that avoid the flapping router, these SPF recalculations should not cause any instability. Only if the CPUs on routers become so busy they can not even keep up with sending and receiving OSPF Hello packets in a steady manner, the entire network could become unstable because the OSPF adjacencies would globally start falling apart and come back. This is a rather extremal scenario, though, that would be very difficult to demonstrate with today's routers' CPU horsepower.

It shall also be mentioned that various vendors have implemented different throttling mechanisms to prevent OSPF from engaging into rapid repeated SPF recalculations. This is usually done by progressively doubling the time necessary between two subsequent SPF runs, with the first run being scheduled very soon and subsequent runs are exponentially delayed.

Best regards,

Peter

OSPF question about intermittent line interruption

To add to Peter's excellent post, I would also like to mention, that from each router's perspective a Shortest Path Tree will be formed to all other routers within the area. This is based off of information found in the LSDB. Mostly Type-1 and Type-2, and Type-1 has the most topological informatiom about OSPF.

If you have let's say, a router in Area 50, then you are going to have a partial SPF run, if you are in another area, because if you are connected to Area 10, which is connected to 0, and then 50, and the router in 50 has issues, from Area 10's point of view, the other networks (Type-3 and Type-5) are leaf nodes. So you will only have a partial SPF run, instead of a full spf run.

There are features such as LSA Group Pacing, Incremental SPF, etc, that can help with SPF calculations.

There is also another feature, which I forgot, which is kind of a route dampening technology as you would see with BGP, but with OSPF to a certain extent.

New Member

OSPF question about intermittent line interruption

Thanks John.

New Member

OSPF question about intermittent line interruption

Thanks Peter.

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