Basis question on OSPF Area range command

Answered Question
Feb 9th, 2010

Dear friends,

Lets say that we have a topology where A and B are ABR's and send Type 3 LSA's to router C that is OSPF adjacent to them on Area 1.

So, the topology is roughly like this:


   Area 0 
A -------->B

   Area 1 
A -------->C

   Area 1 
B -------->C


Now lets talk about just A and C.

Should area range command on A be used to summarize:

a. Type 3 LSA's advertised by the ABR A to C in another area

(or)

b. Not all Type 3 LSA's but only Routes installed by C that point to A as next hop. In other words, if both A and B are each advertising 10 Type 3 LSA's to C, and C installs only 5 of them pointing to A as next hop, then should A summarize on these 5 routes only or on all the 10 Type 3 LSA's?


Thanks a lot
Gautam


I have this problem too.
0 votes
Correct Answer by Edison Ortiz about 6 years 11 months ago

The behavior of area range on ABR A will depend on which area are you summarizing.

If you perform an area 0 range, ABR A will summarize routes from Area 0 into Area 1 - per your example.

If you perform an area 1 range, ABR A will summarize routes from Area 1 into Area 0 - per your example.

Both conditions are irrevelant to the advertising router shown in the RIB as both ABRs have the same OSPF database.

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Correct Answer
Edison Ortiz Wed, 02/10/2010 - 07:33

The behavior of area range on ABR A will depend on which area are you summarizing.

If you perform an area 0 range, ABR A will summarize routes from Area 0 into Area 1 - per your example.

If you perform an area 1 range, ABR A will summarize routes from Area 1 into Area 0 - per your example.

Both conditions are irrevelant to the advertising router shown in the RIB as both ABRs have the same OSPF database.

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