How to determine areas OSPF number limit on a 3750

Hi guy, I wanna to know the limit of a 3750 as a area border router : how many areas can it support without CPU problems when a topology change appears

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the bad news - you probably dont want to hear this but - it depends.

CPU load in steady state depends on the size of each area (routes, router number and types of routers) and background processing need (number of areas, neighbours and hello timers).

topology change load depends on size of an area, size of the "hit", amount of change that goes across the area boundary.

there is also a big scaling factor where a major change replicates through the number of alternate paths thru an area - the effect depends on where a device is in the area and scales by some power of the number of loops.

the other obvious Q is what do you mean by CPU "problems" - the limit depends on what else the box has to do....

the good news is that IOS has some OSPF control timers that limit the CPU load by limiting the number of dykstras that can occur. The timers use exponential backoff if you get flapping links and other recurring topology changes to limit the damage.

however - pushing the limits on something like this is a real bad idea, since the 1st thing that happens if you get well past the "edge" is an unstable network.

If you build it right initially, but the convergence time starts to degrade, you are getting close to the limits somewhere - probably in an ABR.

so - minimise the number of areas on a box, and keep the area design clean, using aggregation at ABRs, and limit the use of ASBRs. virtual links are generally a bad idea as well.

if you have the choice use more powerful boxes as ABRs, or where the router needs to handle lots of neighbours.

finally - once it is up and running, test it by causing big topology changes and make sure it is stable.

have a look at:

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Thanks for your answer, have a good day

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