Stephen, Chris,
The minimum value of 2 configurable on the "neighbor ebgp-multihop" command
will allow you to connect to a router that is one or two hops away. I
presume that when ebgp multihop was introduced the assumption was that the
non connected peer would be at least two hops away. Hence the minimum value
of 2 for the "ebgp multihop" command.
At 07:43 PM 6/20/2002 -0500, Stephen Gill wrote:
>I'm not quite sure what topology you are referring to here but at the
>end of the day, I don't think it really matters that ebgp-multihop is 2
>or 5 or 10. All you are doing is limiting the scope of your EBGP
>visibility on a hop count basis.
>
>-- steve
>
>-----Original Message-----
>From: chris [mailto:chris@chrisland.net]
>Sent: Thursday, June 20, 2002 11:30 AM
>To: Stephen Gill
>Cc: cisco-nsp@puck.nether.net
>Subject: Re: [nsp] eBGP multihop - how to count?
>
>dear,
>
>i understand why i need multihop=2 for 2 routers case. however this
>logic does not apply when i have 3 routers, where BGP can still be up
>for multihop=2. shouldn't no. of hops = 3?
>
>chris
>
>Stephen Gill wrote:
> >
> > Consider the loopback as being routed, thus the ttl is decremented
> > since they are not on the same network. IE. Source IP of the packet
> > shows up as the loopback IP, not the directly connected interface, and
> > Dest IP of the packet shows up as the remote loopback IP, not the
> > directly connected interface.
> >
> > -- steve
> >
> > --- chris <chris@chrisland.net> wrote:
> > > yup. but it can't explain the case when 2 routers are connected
> > > back-to-back but "ebgp-multihop 2 (or larger)" is still necessary to
> > > bring up BGP with loopbacks.
> > >
> > > chris
> > >
> > > Stephen Gill wrote:
> > > >
> > > > I believe this directly corelates to the TTL set in the packet.
>It
> > > is
> > > > counted just like the TTL would be when routing packets. IE,
> > > decrement
> > > > the TTL by one at every hop.
> > > >
> > > > -- steve
> > > >
> > > > --- chris <chris@chrisland.net> wrote:
> > > > > Dear all,
> > > > >
> > > > > When I have 2 routers connected back-to-back and eBGP peered
>with
> > > > > their
> > > > > loopbacks, eBGP can only be up with eBGP-multihop >= 2.
>However,
> > > > > when
> > > > > these 2 routers are connected via another router, eBGP can still
> > > be
> > > > > up.
> > > > > E.g.:
> > > > >
> > > > > Router A <--- eth ---> Router X <--- eth ---> Router B
> > > > >
> > > > > eBGP between A and B's loopbacks can be up with eBGP-multihop =
> > > 2.
> > > > >
> > > > > Isn't it "3"? How is it calculated?
> > > > >
> > > > > Thanks.
> > > > >
> > > > > Regards,
> > > > > Chris
> > > > >
> > > >
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Harold Ritter, CCIE 4168
Advanced Network Services - ISP East
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824 USA
Phone: 978 497 3129
Fax: 978 497 3129
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