OSPF multi-level hierarchy: Necessary at all?

Jessica Yu jyy at ans.net
Fri May 28 18:51:40 UTC 1999

>Btw. Flat network...
>Routers now have - 128 or 256 MB RAM, 300 - 400 Mhz CPU. I guess you can 
>built flat betwork with 5,000 routers withouth hard problems.

With a network this large, probability of any router interface going down, 
link flapping or rotuer failure is high so it can happen frequently.  With 
one giant flat IGP area, any such incident occuring anywhere in the network 
will cause network wide LSA flooding, SPF calculation and link database update.
Lots of network resources including router CPU will be consumed.  Even one 
has a lot of cpu power in super routers, it's still a good idea to prevent an 
outage anywhere in the network from causing network wide event i.e. to 
localize the problem.  A giant flat IGP area in a large network does not help 
achieving fault isolation.


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