Sina Mirtorabi wrote:

> Acee,
>
> Acee Lindem wrote:
>
> > Nehal Bhau wrote:
> >
> > > Hi folks,
> > >
> > > Consider the following simple network topology.
> > >
> > > N1
> > > N2
> > > R1--------------R2--------------|
> > > Area 0
> > > Area 1
> > >
> > >
> > >
> > > R1 is an ASBR configure to advertise a route (AS External LSA)
> > > to some network N3 with next-hop set to a non-OSPF router on network
> > > N2.
> > >
> > > The question is, should R1 set the forwarding address to be the
> > > address
> > > on network N2 ? It seems R1 will have an inter-area route to network
> > > N2.
> >
> > If R1 is attached to Area 1, it is an intra-area route.
> >
> > > So, it should set the forwarding address (which is an address on N2).
> > >
> > > Is that correct ?
> >
> > If R1 has an OSPF intra-area or inter-area route to N2, then it should
> > set the forwarding address.
> >
>
> the ASBR that is originating type 5 LSA in charge of setting the forwarding
> address in our case R1, is directly connected to the nexthop ( forwarding
> address ) so it can never have an Inter area route to this forwarding
> address.
> however once the forwarding address is set for type 5 other route must have
> an intra or inter area route to the forwarding address.
>
> Nehal
>
> other condition to set the forwarding address ( although it is not
> explicitly mentioned ) is that the interface to the next hop should be
> Multi-access since setting it on p2p is equivalent to setting it to zero (
> we assume that OSPF is enabled on the interface )
>
> Sina
>
> >
> > >
> > > Thanks,
> > >
> > > -Nehal
> > >
> > >

I am sorry, I think the network diagram is not very clear.

Here it is,

R1----N1(Area 0)-----R2----N2 (Area 1)-------|

R1, R2 are OSPF routers.
N1, N2 are networks.

R1 and R2 are connected via network N1 in area 0.
R2 has another OSPF enabled interface in network N2, part of area 1.
R1 does not have an interface on network N2.

So, R2 will summarize network N2 to area 0. So, R1 should have an
inter-area route to N2. Now, the forwarding address is an address on
network N2. So, R1 can reach network N2 via the inter area route.

So, I think Acee is right, since R1 has an inter-area route to reach the
forwarding address, it should set the forwarding address (which is on N2)
correctly.

Although a lot of implementations seem to be setting the forwarding address
to 0.0.0.0 in this scenario.

-Nehal

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