ospf route summarization problem

Unanswered Question
Apr 24th, 2007

I have three routers:



R1- area 0 -R2- area 1 -R3- area2

__________X---virt link---X_________



There is a virtual link configured over area 1 on R2 and R3.

R1 has 8 ospf interfaces with contiguous IPs 60.60.X.3 X=0..7

R2 has a range configured to condense them from area 0 into area 1:

router ospf 1

area 0.0.0.0 range 60.60.0.0 255.255.248.0



When the virtual link is up, R2 redistributes ALL 8 routes as LSAs into area 1! (seen with show ip ospf database)

When I disable the virtual link on R3, R2 does the condensation and only 1 LSA is advertised into area 1.




Do you know what document states that if on an ABR like R2, if the destination area like area 1 has a virtual link up in it, the routes are NOT condensed INTO it? (this seems to be the exact cisco behaviour)



There might be some common sense logic that when the virtual link is up, all routers are in area 0 and no condensation is needed, but I would like to understand the theory also.


Thank You!

Ion Petrescu

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sundar.palaniappan Tue, 04/24/2007 - 16:05

Ion,


I don't have a link for you explaining this behavior. But, I am sure there has got to be an RFC or Cisco document that has this documented.


I can try to explain this behavior though. What you are seeing is the normal behavior. By virtue of having the virtual-link R3 becomes an ABR. As you may know, all OSPF routers in an area would have an identical database. As both R2 and R3 are connected to the backbone (area 0) if R2 were to summarize the route(s) to R3, when the virtual link is up, that would violate the OSPF requirement of an identical database of area 0.


When you do not have a virtual link the 'area range' command works as it supposed to by summarizing the route into area 1.


Hope that makes sense :)


HTH


Sundar

ipetrescu Tue, 04/24/2007 - 16:30

Thank you, but:

<>


R2 would summarize the route(s) into area 1, not area 0! (area 0 can have all the 8 LSAs and no summarized LSA on all routers)


For me, it would be logic that R2 would summarize these 8 routes it installs in its routing table into one LSA in area 1.

But this does not happen. From what I see, R2 refuses to summarize AND it is the only one who in fact ORIGINATES all those 8 LSAs into _area 1_!

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