Wu,

That advantage of aggregation is that you consume less network bandwidth
(during advertisements), less memory(for storing routes) and less CPU
(during routing table calculation). However, this condensation of
information could result in suboptimal routing.

Summarization is performed on area boundaries (OSPF runs a link-state
protocol within each area and a distance-vector protocol between areas. This
makes the protocol more scalable.) and during redistribution from other
routing protocols, like BGP. Such redistributed routes are advertised using
External-LSAs.

Thanks,
Balaji.R.

Hi again,

Thank you Acee Lindem and Balaji for your informative comments. I have more
questions.

1) What are the advantages and disadvatanges of route aggregation?
2) On what basis do we need to use route aggregation?
3) I believe route aggregation process will cause ambiguous link metric(s).
How critical will this ambiguity cause?

Further question below:

>>Given that a router has the following fictious routing information:
>>
>>i) prefix = 10.2.153.178/32; next-hop = router2
>>ii) prefix = 10.2.153/23; next-hop = router2
>>iii) prefix = 10.2.154/23; next-hop = router2
>>iv) prefix = 10.2/16; next-hop = router2
>>
>>The question is, is it possible to aggregate them such that we have >>only
>>the following prefix in the routing table?
>>
>>iv) prefix = 10.2/16; next-hop = router2
>>
>>Are you talking about External-LSA's here? If so, it is better not to
>>aggregate as the intent of the sender might to update just the LSA
>>that changes. (Otherwise, the sender would have probably aggregated
>>the routes). If you aggregated, you wouldn't be able to do
>>incremental routing table calculation.

I am a newbie in OSPF, but I would appreciate a brief explanation about
external-LSA. Does route aggregation process only allowable on external-LSA?

Thank you in advance.

-Tze Ven

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