Doubt in FA of NSSA LSA

Unanswered Question
Jul 31st, 2007

Hi all,

As in the diag attached,

I am running ospf in R1,R2,R3 and running BGP ins R3 and R4, where R2 and R3 are in NSSA.

I want more explanation for the excerpt from RFC3101

"Normally the next hop address of an installed AS external route

learned by an NSSA ASBR from an adjacent AS points at one of the

adjacent AS's gateway routers. If this address belongs to a network

connected to the NSSA ASBR via one of its NSSAs' active interfaces,

then the NSSA ASBR copies this next hop address into the forwarding

address field of the route's Type-7 LSA that is originated into this

NSSA, as is currently done with Type-5 LSAs."

Consider the IP interface address of R4 connected to R3 is 10.20.30.1 and R4 is advertising 200.100.0.0.

In that case the next hop address for 200.100.0.0 in R3 would be 10.20.30.1. My doubt is how can it belongs

to to a network connected to NSSAs' active interfaces ? is there any chance , pls explain with an example ?

Thanks,

Vijaybabu

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Harold Ritter Tue, 07/31/2007 - 06:26

Vijaybabu,

The interface between R3 and R4 will be active in the NSSA if there is a network statement linking it to the NSSA area and if no "passive-interface" statement is configured for it.

Hope this helps,

GillieLucent Tue, 07/31/2007 - 08:33

Hi,

If you mean the network statement which is used to identify interfaces running ospf, in that case the interface in R3 which is connecting to R4 should run OSPF. But in my case its running BGP. In that case, what would be the explanation for my earlier question ? Is it possible of R3's interface running OSPF, but R4's interface running BGP ?

Thanks,

Vijaybabu

Harold Ritter Tue, 07/31/2007 - 09:12

Vijaybabu,

You could have a network statement for the interface between R3 and R4 under "router ospf" on R3 for the purpose of having that IP subnet propagated via OSPF and at the same time run BGP between R3 and R4. No adjacency will be formed between R3 and R4 as R4 doesn't run OSPF.

The interface would then be considered active in the OSPF NSSA area and the FA would be set to the IP address of R4.

Hope this helps,

sundar.palaniappan Tue, 07/31/2007 - 11:22

There's another option for you to address the problem with FA in a NSSA setup. You can configure the command, "area area-id nssa translate type7 suppress-fa" on the ABR to suppress FA and this way the router that translates the type7 to type5 LSA would use 0.0.0.0 as the FA.

HTH

Sundar

s.arunkumar Tue, 07/31/2007 - 23:14

interesting...

as sundar mensioned ,supress-fa can be used when u dont have reachablity towards FA.

also something about FA of type 5 lsa(just for info)...

a router wont install the route learned via type 5 lsa if the FA of the route is also learned via type 5 lsa...

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