The OSPF RFC (1583) did not specify any guidelines for the number of routers in an area or number the of neighbors per segment or what is the best way to architect a network. Different people have different approaches to designing OSPF networks. The important thing to remember is that any protocol can fail under pressure. The idea is not to challenge the protocol but rather to work with it in order to get the best behavior. The following are a list of things to consider.
1. Number of Routers per Area ?
>>The maximum number of routers per area depends on several factors, including the following:
a.What kind of area do you have?
b.What kind of CPU power do you have in that area?
c.What kind of media?
d.Will you be running OSPF in NBMA mode?
e.Is your NBMA network meshed?
f.Do you have a lot of external LSAs in the network?
g.Are other areas well summarized?
For this reason, it's difficult to specify a maximum number of routers per area.
2.Number of Neighbors ?
3.Number of Areas per ABR ?
4.Full Mesh vs. Partial Mesh ?
5.Memory Issues ?
Some of the benefits of OSPF are, fast convergence, VLSM, authentication, hierarchical segmentation, route summarization, and aggregation which are needed to handle large and complicated networks.
Thanks for your info on maximum number of routers per area. I would like to know the maximum number of routing updates (number of LSAs) OSPF can handle effeciently. I heard from many that OSPF can be prescribed for routing updates less than or equal to 7000.
As it has already been mentioned with other words, OSPF does not run on its own inside its private chamber or together with its specification. It runs on real machines with CPU, memory, bandwidth and other characteristics. However, you are right that there are some limitations imposed by the specification that have been mentioned by Moy (the RFC author). I will look them up and I will inform you (my memory faints with time). For the time being, I do know that the number 7000 that you mentioned is not a specification imposed limitation, because if you have the resources (not particularly fancy resources actually), you can keep a stable network with more than 5000 externals. I am counting the externals only. There can be many more LSAs by other sources besides redistribution, so the number that you heard of is definitely not a specification imposed limitation.
There is also some information about IS-IS limitations there that you might find interesting.
Besides that, it is supposed to be possible to configure some theoretical limit in the number of external LSAs in a domain(ospfExtLsdbLimit), which is described in RFC 1765 (OSPF Database Overflow, http://www.ietf.org/rfc/rfc1765.txt). This "limitation" has to do with the fact that "Proper operation of the OSPF protocol requires that all OSPF routers maintain an identical copy of the OSPF link-state database.". However, this feature is not included in RFC 2328. Probably because there are other ways (area types that limit externals) to force those low resource routers out of the way.
I am almost sure I had once come across a limitation regarding external routes, but unfortunately I cannot find the information right now (although this issue is described in the first URL above, I thought I had come across an analysis by Moy or Radia Perlman, but then again, my memory faints with time as I have already said). In any case, you would not redistribute the full Internet Routing Table into OSPF, would you? :-)
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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