OSPF NSSA Question

Unanswered Question
Aug 10th, 2010

Hi!

I wonder why I get such a route on my ABR:

O*E1 0.0.0.0/0 [110/10067] via 10.47.0.7, 00:05:52, Serial0/0

This router is connected to backbone area and area 1. Serial 0/0 is assigned to area 1 nssa only.

Also, do I have to have only one exit to the backbone area or may I have two exits from NSSA (and how about stub and totally stubby?). Because on the other ABR router with backbone area, I have the wished route:

O*N1 0.0.0.0/0 [110/10002] via 10.67.89.8, 00:28:19, Serial0/0

Thank's,

Roberts

I have this problem too.
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Richard Burts Tue, 08/10/2010 - 10:11

Roberts

You may have more than one exit (more than one ABR) for NSSA, stub, and totally stub areas. And I would advocate that having more than one exit was a good thing.

I am not sure about the route in your question. Perhaps if you post the output of show ip interface brief and of show ospf then we might get a better idea of what is going on.

HTH

Rick

francisco_1 Tue, 08/10/2010 - 10:42

I wonder why I get such a route on my ABR:

O*E1 0.0.0.0/0 [110/10067] via 10.47.0.7, 00:05:52, Serial0/0

You are seeing this proberbly because on 10.47.0.7, you are adversting a default route with a metric-type 1 in to OSPF

some like "default-information originate metric-type 1"

Francisco.

Giuseppe Larosa Tue, 08/10/2010 - 12:03

Hello Roberts,

you can have two ABR nodes between an OSPF NSSA area and backbone area but only one perform the LSA translation from LSA type 7 (external routes within NSSA) and O E1 or O E2 in area 0 (and all other non stub areas in domain).

I may be wrong but my guess is that within NSSA area an ASBR router is injecting a default route as O N1 with IP next-hop 10.67.89.8

O*N1 0.0.0.0/0 [110/10002] via 10.67.89.8, 00:28:19, Serial0/0

the ABR receiving this external route wins the competition to translate the LSA type 7 into an LSA type 5 ( to be noted an O N1 is converted in an O E1).

As a result of this it is the only one to generate an O E1 LSA for 0.0.0.0/0 that will take in account the total cost on path so:

a serial interface with default reference bandwidth costs 64, 1 is the cost of a 100 Mpbs or above

O*E1 0.0.0.0/0 [110/10067] via 10.47.0.7, 00:05:52, Serial0/0

if 10.47.0.7 is a link to other ABR but in area 0 not in NSSA, my guess can be correct

the other ABR that has losed the competition installs in its routing table the O E1 route not the O N1 as it prefers a route in area 0 to one in NSSA area.

Hope to help

Giuseppe

ropakalns Tue, 08/10/2010 - 13:58

Hi!

Oh, I did not know that only one ABR can translate from Type7 to Type5 (and vice versa, too?). Then it makes a lot of sense, except the fact that my serial link is a part of area 1 (NSSA).

I attached a picture - there you can see the topology R8 gives the default route as Type7, R6 is giving me the default route towards NSSA as O*N1 and then R4 gave me O*E1 towards R7. NSSA does not have LSA Type5, right? So why E1 towards R7?

How does OSPF determine which ABR to choose for LSA Type7 to Type5 translation?

And if I assume that there will be injected two default routes from two directions (redistributed through NSSA and from backbone or other normal area), what decision will the routers in NSSA make if there will be two equal cost routes?

Unfortunately, my GNS3 did not shut down properly so I cannot reconfigure all this stuff so quickly. But there are too much questions for me so I'll probably build it once again.

Thank's!

Roberts

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Giuseppe Larosa Tue, 08/10/2010 - 14:17

Hello Roberts,

>> How does OSPF determine which ABR to choose for LSA Type7 to Type5 translation?

highest OSPF router-id between the two ABR nodes

>> Note that a Type-7 default LSA originated by an NSSA border router is

   never translated into a Type-5 LSA, however, a Type-7 default LSA
   originated by an NSSA internal AS boundary router (one that is not an
   NSSA border router) may be translated into a Type-5 LSA.

http://www.faqs.org/rfcs/rfc3101.html


 3.1 Type-7 Translator Election

>> An elected translator will continue to perform translation duties
   until supplanted by a reachable NSSA border router whose Nt bit is
   set or whose router ID is greater.

the RFC 3101 provides a more general election process that includes
a new bit Nt in router LSA that each ABR generates into the NSSA area.
However, in case of a tie on Nt bit value the highesr OSPF RID wins.

Hope to help

Giuseppe

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