Hi Manav,
   Thanks manav for reply
   but i have one more doubt ...
   If the forwarding address is a router and it
belongs to another AS,then what is in the case of
first time calculation of ASExternal LSAs.
 We will not have any routes to any other AS.Then we
will never find a route to Forwarding address in the
Routing table.
Then how will we proceed if we don't have any entries
for the forwarding address in the Routing table

alex
--- Manav Bhatia <manavSAMSUNG.CO.KR> wrote:
> 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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