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Funny story: OSPF

While working in a Support Organization of a networking company, I come across a lot of funny and interesting stories. Here is one of them.

One of our customers, who's name we of course will not mention, opened a Support case. He complained that his routers were suffering from a high CPU load and thus the routing table was not up-to-date. Very quickly we came to the thought that is had to be because of OSPF not updating the hardware table quickly. At some point I asked him how many OSPF routers he had in the Area. His answer was 1500!!!

One might say that 1500 routers in 1 OSPF area can occur in large networks. This sure can happen, but then it is time for a redesign of the network. Let me explain why this isn't a good idea.

OSPF calculates the shortest path for each route. This means that if 1 router goes down, it will need to re-calculate the shortest path. With 1500 routers in 1 Area it will result in a lot of OSPF changes and thus a lot of re-calculations. Add all packets sent to the OSPF All-Routers Multicast group and you will have a swamped CPU.

What would be a good number of routers in 1 Area then? There is no clear answer for that. When asking around a number between 200 and 250 seem to be common. Keep in mind that there are routers with a stronger CPU and routers with a smaller CPU (mostly elderly routers). Remember that even the smaller sized routers need to be able to update their tables, otherwise they can choke the rest of the network. If they can't keep up with the rest, place them in a Stub Area or replace them.