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From: Subhasree Mandal (sumandal_at_NORTELNETWORKS.COM)
Date: Fri Sep 06 2002 - 16:29:11 CDT
I have a question about type-7 LSA origination and route calculation.
Let us consider the following configuration
|--cost 10-------R3--------cost 10----|
Lets consider that all these routers are in an NSSA area nd R3 is an ASBR
advertising an external
route to network Nx. If we want that route to be translated to type 5 by
some ABR in this NSSA area, we must set the forwarding address to a non-zero
value (page 6, RFC 1587). According to page 7 of
the same RFC
"...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
So my type-7 LSA for Nx can have forwarding address as the interface address
of R3 on network N3.
When R1 computes route to Nx, it would actually use the next-hop for N3 as
the next-hop for Nx since
the forwarding address is in N3. So if R1 chooses to reach N3 through R2
(equally possible as through
R3), the packets from R1 would reach R3 (to go out of the AS) via R2. In
other words it is adding
an extra hop to the route to Nx.
How do we address this issue?