OSPF NSSA forwarding address selection

Unanswered Question
Aug 28th, 2007
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I noticed this bizarre behaviour and wondered if anyone has encountered this before and know why this is happeneing.

The scenario is as follows:


Each router has a loopback address corresponding to its router number. For example

R1's loopback 0 ip address is in area 0

R2 is in area 0

R3 is in area 51 NSSA

[router id's are configured in the OSPF process of all routers]

R4 is in RIPv2

The LAN is in area 0. The WAN is in area 51 NSSA. The WAN is running RIPv2. R4's loopback is in RIP.

All routers are 7206 VXR NPE400 running 12.4(12) service provider feature set. I have also test this scenario with 2600 [ip plus] and 2800 routers.

On the NSSA ASBR [R3] which is rediatributing RIP into OSPF, it will redistribute with the forwarding address of it's router id because the WAN is not enabled in OSPF. This bit I understand and expect.

The wierd bit is that once you configure a new loopback on R3 and add it to the OSPF process, the forwarding address of the NSSA external LSA is changed to that address immediately!!! IT TOTALLY IGNORES THE ROUTER ID THAT WAS CONFIGURED!!! If you add another new loopback and add it to OSPF it changes the forwarding address again!!![this problem only affect NSSA ASBRs]

Anyone know why this is??

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Overall Rating: 5 (2 ratings)
mheusing Tue, 08/28/2007 - 05:02
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from RFC 1587 "The OSPF NSSA option":

"If the network between the NSSA AS boundary router and the adjacent AS is advertised into OSPF as an internal OSPF route, the forwarding address should be the next hop address as is currently done in type-5 LSAs, but unlike type-5 LSAs if the intervening network is not advertised into OSPF as an internal OSPF route, the forwarding address should be any one of the router's active OSPF interface addresses."

So the forwarding address is a real IP on R3. The router ID could be nonexistent in the routing table (f.e. OSPF router ID and thus not usable in this context.

Hope this helps! Please rate all posts.

Regards, Martin

P.S.: Most RFCs are understandable and often answer this kind of questions right away. I found, it often added to my understanding to read the thoughts contained in the RFCs.

cjnwodo Tue, 08/28/2007 - 05:17
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Thanks for the swift response, however, it looks like Cisco took the wording,"...the forwarding address should be ANY one of the router's active OSPF interface addresses." quite literary. Because it always picks the newest loopback IP address. And then the NSSA ASBR will then resend all the NSSA-external LSAs with the new loopback address as the forwarding address. Even if the new loopback was a lower or higher number it didn't matter, it still changed the forwarding address.

I believe that this behaviour by the Cisco router is wrong, it should stick with the configured router id.


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