From: Narbik Kocharians (narbikk@gmail.com)
Date: Fri Feb 23 2007 - 04:10:35 ART
Well, neighbor next-hop-self is also needed between the EBGP neighbors when
the connection is multipoint and the spokes don't have reachability to each
other, especially in a frame-relay hub and spoke where all routers are from
the same IP address space.
Let's say R1 is the hub and R2 and R3 are the spokes, and R2 and R3 have an
EBGP peer session to R1. When R2 advertises a prefix to R1, it will set the
next hop to be it's own IP address (R2's IP address), when R1 advertises the
same prefix to R3, since the interface is a multipoint interface and the
neighbors (R2 and R3) are from the same address space, R1 does not change
the next hop attribute, therefore the spokes (R2 and R3) will not have
reachability to each others IP address, one way to fix this issue is to
configure R1 to change the next hop attribute for it's clients (R2 and R3).
On 2/22/07, Dwi C Taniel <dc@dwichandra.info> wrote:
>
> ... and here is the example if you needed it (at least I got the idea
> better with this example)
>
> R1 (AS1) --- R2 (AS2) --- R3 (AS2) --- R4 (AS2) --- R5 (AS3)
> The 'neighbor x.x.x.x next-hop-self' would need to be in R2 and R4 for
> their neighbourship with R3 (yes, in some countries they spelled it
> 'neighbour', which give me problem in configuring bgp in the past.
> Thanks to the 'neigh' shortcut that remove this problem)
>
> Thus, R3 would be informed by R2 and R4 for reachability to R1 (AS1)
> and R5 (AS3).
> Without 'next-hop-self', R3 would receive the NLRI (CMIIW for the
> terms as I am still a barbaric network engineer) but did not have the
> 'best' selected (the one with symbol '>' from 'show ip bgp' in R3)
>
> Hope it helps ;)
>
> DC
>
> ========
> http://www.dwichandra.info
> dc@dwichandra.info
> --------
> visit Transformers movie link at http://www.dwichandra.info/transformers
> --------
>
>
> On 02/23/2007, "Schulz, Dave" <DSchulz@dpsciences.com> wrote:
>
> > Dennis -
> >
> > When a route is received via ebgp, and passed into ibgp....the next hop
> > is not manipulated. Therefore, the routers within the AS will not know
> > how to reach this next hop (learned by ebgp).
> >
> > By using the next-hop-self command, the AS border router is set as the
> > next hop. This allows the other routers within the AS reachability,
> > since the border router knows how to reach the route on the next AS.
> >
> >
> > Dave Schulz,
> > Email: dschulz@dpsciences.com
> >
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> > dennis lin
> > Sent: Thursday, February 22, 2007 10:13 PM
> > To: ccielab@groupstudy.com
> > Subject: BGP --neighbor x.x.x.x next-hop-self
> >
> > I am quite confused about this BGP command: neighbor
> > x.x.x.xnext-hop-self Do you know under what situation I have to
> > issue this
> > command? Thanks
> >
> >
> > Dennis
> >
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>
>
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-- Narbik Kocharians CCIE# 12410 (R&S, SP, Security) CCSI# 30832 Network Learning, Inc. (CCIE class Instructor) www.ccbootcamp.com (CCIE Training)
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