From: Marvin Greenlee (marvingreenlee@yahoo.com)
Date: Mon Jan 09 2006 - 18:12:18 GMT-3
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.
> > >
> > >
> >
>
This archive was generated by hypermail 2.1.4 : Wed Feb 01 2006 - 07:45:48 GMT-3