From: CCIEin2006 (ciscocciein2006@gmail.com)
Date: Mon Jan 09 2006 - 18:18:42 GMT-3
Okay,
But if you're not specifically configuring local AS on the neighbor
statement to R1 would that local as number still appear?
On 1/9/06, Marvin Greenlee <marvingreenlee@yahoo.com> wrote:
>
> Perhaps "downstream" would be a better word. It
> depends on the direction of the networks learned with
> regards to the local-AS.
>
>
> In the example given, R3's peering to R4 includes the
> local-as keyword.
>
> Networks learned in the Direction R1 to R5 -->
> (R4 and R5 learning about the network on R1.)
> Since R4 is peering to the local-AS, all routes
> learned from R3 need to have the local AS in the path,
> or R4 won't know where to send the traffic.
>
> Networks learned in the Direction R5 to R1 <---
> (R1 and R2 learning about networks from R4 and
> beyond.)
> In the other direction, R1 and R2 don't necessarily
> need to know about the local-AS configured on R3. R2
> knows that it needs to get to AS4, and that it can get
> to AS4 via R3. Since R1 and R2 don't need this
> information, R3 has the option to "no-prepend" and not
> add the local AS to the path.
>
> --- CCIEin2006 <ciscocciein2006@gmail.com> wrote:
>
> > Hello Marvin,
> >
> > What constitutes "behind the local-as". Do you mean
> > a confederation or do
> > you mean another ebgp peer?
> > In the case of ebgp peer isn't it all relative i.e.
> > from the perspective of
> > R1, R4 is behind AS23?
> >
> >
> > On 1/9/06, Marvin Greenlee
> > <marvingreenlee@yahoo.com> wrote:
> > >
> > > No-prepend example from another message board:
> > >
> > > ...
> > > 'no-prepend' prevents the 'local-as' AS from being
> > > added to the path, but only for peerings "behind"
> > the
> > > 'local-as' peering. From the outside, you will
> > always
> > > see the 'local-as'.
> > >
> > > ****
> > >
> > > R1(AS1)--R2(AS23)--R3(AS23)--R4(AS4)--R5(AS5)
> > >
> > > Network XX on R1, Network YY on R5.
> > >
> > > Assuming that R3's peering to R4 includes local-as
> > 100
> > >
> > >
> >
> R1(AS1)--R2(AS23)--R3(AS23)--local-AS100))-R4(AS4)--R5(AS5)
> > >
> > > R1 sees network YY with path 23 100 4 5.
> > >
> > > with no-prepend on R3's peering to R4, R1 would
> > see 23
> > > 4 5.
> > >
> > > R5 sees network XX with path 4 100 23 1.
> > > with no-prepend on R3's peering to R4, R5 still
> > sees
> > > the path as 4 100 23 1.
> > >
> > > ****
> > >
> > > In this case, R1 and R2 are "behind the
> > 'local-as'",
> > > and may or not see the local-as in the path
> > depending
> > > on whether or not the no-prepend keyword is used.
> > R4
> > > and R5 are "outside the 'local-as'", and will
> > always
> > > see local-as, regardless of whether or not R3 is
> > > configured with the 'no-prepend' keyword.
> > >
> > >
> > >
> > > --- Victor Cappuccio <cvictor@protokolgroup.com>
> > > wrote:
> > >
> > > > Hello List..
> > > >
> > > >
> > > >
> > > > On
> > > >
> > > <
> > >
> >
>
> http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/ipr
> > > > rp_r/ip2_n1g.htm#wp1037846>
> > > >
> > >
> > >
> >
>
> http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/iprr
> > > > p_r/ip2_n1g.htm#wp1037846
> > > >
> > > > We can find the description for what is used the
> > > > local-as option in BGP,
> > > >
> > > >
> > > >
> > > >
> > > > no-prepend
> > > >
> > > > (Optional) Configures the router to not prepend
> > the
> > > > local autonomous system
> > > > number to any routes received from an external
> > peer.
> > > >
> > > >
> > > >
> > > >
> > > > Is this correct?
> > > >
> > > > IMHO, I believe that this should be read like:
> > > > Configures the router to not
> > > > prepend the local autonomous system number to
> > any
> > > > routes advertised to
> > > > external peers
> > > >
> > > >
> > > >
> > > > Is this correct or I have a misunderstanding of
> > the
> > > > concept?
> > > >
> > > > Thanks
> > > >
> > > > Victor.
> > > >
> > > >
> > >
> >
> _______________________________________________________________________
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> > >
> > >
> > >
> > >
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> >
>
>
>
>
> __________________________________________
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