Hi Alex,
Yes .. Whenever the forwarding address is non zero it always points to a
router belonging to another autonomous system (refer rfc 2328 footnote [24])
. Whenever this address is non zero a look up is performed on the forwarding
address in the routing table. The matching routing table entry must specify
an inter-area path and if no such path exists, then the LSA is ignored.

Consider the setup as shown.

                     |
 Router A --- -|
 (AS y) |----- Router C (AS x)
                     |
  Router B ----|
(AS y) |

(1) Routers A and B are in the same autonomous system (AS y) while Router C
is in a different autonomous system (AS x).
(2) Router A and Router C are EBGP peers.
(3) Router C may or may not speak OSPF while Routers A and B are both OSPF
routers.

Router A now has to originate AS-external-LSAs for all the destinations it
learnt from Router C. Router B (or any other router on the LAN) to reach
these destinations will forward the packets to Router A which will in turn
forward all such packets (those destined to AS x) to Router C. Thus Router B
would take an extra hop to reach those destinations. To avoid this the
"forwarding address" field is used. Router A will in its AS-external-LSAs
point this field to the router C. Now packets will be directly forwarded to
Router C.

I hope it makes the point very clear.

Regards,
Manav

----- Original Message -----
From: <alexlerinyahoo.com>
To: <manavsamsung.co.kr>; <OSPFDISCUSS.MICROSOFT.COM>;
<alexlerinyahoo.com>
Sent: Tuesday, August 28, 2001 10:21 AM
Subject: Re: (Reply) About ASExternalLSA

> Hi Manav,
> Sorry for the late reply...
>
> According to u what I understand is that the
> forwarding address is always to a router if it is
> Nonzero value.
> But I didn't understand how to reach another AS with
> this Forwarding address.
> Can u make the concept clear that the forwarding
> address will always be to a router when it is a non
> zero value.
> regards,
> alex
>
>
>
>
>
> --- manavsamsung.co.kr wrote:
> > Hi Alex,
> > Forwarding address is used to optimze the final hop.
> > If the advertising AS is advertising a destination
> > that can more optimally be reached by a different
> > router on the same LAN, then the advertising AS puts
> > that router's address into this field.Otherwise it
> > leaves the field as 0.0.0.0, indicating
> > that packets for the external destination should be
> > forwarded to the advertising OSPF router.
> >
> > Without this field, in certain topologies, a route
> > may traverse an extra LAN hop.
> >
> > Hope it helps.
> >
> > Manav Bhatia
> >
> > ----- Original Message -----
> > From: <alexlerinYAHOO.COM>
> > To: <OSPFDISCUSS.MICROSOFT.COM>
> > Sent: Thursday, August 16, 2001 4:56 PM
> > Subject: About ASExternalLSA
> >
> >
> > > Hi..
> > > I have a basic doubt about ASExternalLSA.
> > > This LSA contain a field called forwarding
> > address.
> > > What is this ...
> > > Can this field be a Router ,other than ASBR or is
> > a
> > > Network..
> > > If this can be a network what will be the field
> > > indicate(DestinationID or linkstateid of
> > > networkLSA)..?
> > > Please clarfy my doubt..

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