OSPF NSSA Area with 2 ABRs and 2 ASBRs

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May 22nd, 2008
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Hi all

I have a problem with a small NSSA Area with 2 ABRs and 2 ASBRs which advertising the same external routes. Both ASBRs are connected to both ABRs. The ABR with the higher ID injects both paths as "N2" in his routing table. But the ABR with the lower ID injects only one path (as "E2") in his routing table!!

Is this an normal behvior, that:

- the ABR with the higer ID generates only

one Type-5 LSA out of the 2 Type-7 LSAs

The "show ip ospf database

nssa-xternal" shows both entries.

The "show ip ospf database external" shows

only one entry

- the ABR with the lower ID "trust" the one

Type-5 LSA more then the both Type-7 LSAs

in the database

Thanks for your help


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tstanik Wed, 05/28/2008 - 09:07
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As part of the NSSA's Dijkstra calculation after type-5 and type-7 routes have been calculated. If the calculating router is not an area border router this translation algorithm should be skipped. All reachable area border routers in the NSSA should now be examined noting the one with the highest router ID. If this router has the highest router ID, it will be the one translating type-7 LSAs into type-5 LSAs for the NSSA, otherwise the translation algorithm should not be performed.

Which seems to means to me that only one ABR (the highest router-ID) does type-7 to type-5 translations. If this is the case then a second (lower router-ID) ABR in a NSSA area would never see any incoming traffic and would only do so if the primary ABR failed

For the further detail following the URL :


tgareis Wed, 05/28/2008 - 10:09
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If you have a look to the attachment.

Both router have 2 NSSA External entries.

But both have only 1 External Entry.

And the lower ID router has the route as E2 in the routingtable.

Harold Ritter Wed, 05/28/2008 - 13:15
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It is normal behavior for the ABR with the higher RID to do the type 7 to type 5 translation (RFC3031). It is also normal behavior for the other ABR to pick the E2 as an E2 is preferred over an N2.


tgareis Wed, 05/28/2008 - 23:16
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Is it then also normal, too generate only one E2 entry out of this 2 NSSA entries?




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