OSPF Area

Answered Question
Nov 14th, 2009

Hi Netpro

I have little knowledge of OSPF but in real life scenario when multiple ospf area is needed

Correct Answer by Joseph W. Doherty about 7 years 3 months ago

Would OSPF multiple areas be effective? Probably. Are they necessary? Hard to say. Again, much depends on actual topology and hardware being used. For instance, is there much of a topology at the HQ or branches? What's the link bandwidth, etc.?

A not uncommon OSPF design might be to make each site it's own area, which allows route filtering. However, for 30 spokes, one area might work fine too. Of course, you place mutiple sites within the same area to decrease the SPF demand per area without the need for numerous areas.

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Joseph W. Doherty Sun, 11/15/2009 - 04:41

The "real life" need for multiple OSPF areas usually relates to larger topologies. Within an area, OSPF uses the SPF (or Dijkstra) algorithm to compute best path for routes. This algorithm can be very CPU intensive. What can happen is routers running OSPF can go into meltdown within a larger area especially if there's any link instability.

You may see rules such as to not exceed 50 routers per OSPF area, but that's not exact. You might need to go fewer, or larger might be fine too. Depends much on what the topology looks like, how capable the routers are (performing this control plane function - BTW, L3 switches often weaker is this aspect vs. a router) and how stable your links are.

The other factor with OSPF, route summarization is done on border routers so areas can be used to control that too.

pshah.1979 Sun, 11/15/2009 - 05:24

Joseph thank you

Site to Site VPN over internet would cross 50 routers or less, is OSPF multiple area effective in this scenario.

Today our company business is Islolated across countries but planning to plug all countries using DMVPN. In total one Hub and 30 Spokes. Not with much experience so still finding hard to conclude either to go ahead with ospf or eigrp. I am aware both can do the job but looking for simple approach with effective performance

Correct Answer
Joseph W. Doherty Sun, 11/15/2009 - 11:52

Would OSPF multiple areas be effective? Probably. Are they necessary? Hard to say. Again, much depends on actual topology and hardware being used. For instance, is there much of a topology at the HQ or branches? What's the link bandwidth, etc.?

A not uncommon OSPF design might be to make each site it's own area, which allows route filtering. However, for 30 spokes, one area might work fine too. Of course, you place mutiple sites within the same area to decrease the SPF demand per area without the need for numerous areas.

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