Gotcha.
-- steve
-----Original Message-----
From: Harold Ritter [mailto:hritter@cisco.com]
Sent: Thursday, June 20, 2002 10:16 PM
To: Stephen Gill
Cc: 'chris'; cisco-nsp@puck.nether.net
Subject: RE: [nsp] eBGP multihop - how to count?
Stephen,
See comments in-line.
At 09:58 PM 6/20/2002 -0500, Stephen Gill wrote:
>One hop = ttl of 1 = no ebgp-multihop no?
>Two hops = ttl of 2 = ebgp-multihop of 2
>etc...
This is right. Except that on a eBGP session, the IOS will check if the
peer address is directly connected. if it isn't, it will expect the
ebgp-multihop command to be configured or else will not attempt to
establish the session. So even if you're one hop away from the peer but
not
directly connected (using loopback), you need to use the ebgp-multihop
command.
>I would presume that when a packet arrives, a router will not worry
>about decrementing a TTL if it is destined to another net on itself (ie
This is correct.
>loopback). Thus, Loopback A on R1 to Loopback B on R2 would only
>require multihop of 2 assuming R1 and R2 are directly connected.
TTL of 1 would suffice to connect to R2 even though the connection is
from
loopback to loopback.
>-- steve
Cheers,
>-----Original Message-----
>From: Harold Ritter [mailto:hritter@cisco.com]
>Sent: Thursday, June 20, 2002 9:54 PM
>To: Stephen Gill
>Cc: 'chris'; cisco-nsp@puck.nether.net
>Subject: RE: [nsp] eBGP multihop - how to count?
>Importance: High
>
>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
>Cisco Systems- "Empowering the Internet Generation."
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
Cisco Systems- "Empowering the Internet Generation."
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