From: Pylko, Eric (EPylko@frontrunnernetworks.com)
Date: Thu Oct 17 2002 - 10:08:15 GMT-3
I believe you have to specify the TTL. The idea is to set it pretty low.
For example, if you're going loopback to loopback you probably want to set
it to 2 (or maybe 3).
The idea is that if the link that connects the 2 routers goes down, you
(probably) don't want to re-route packets from your loopback to the other
routers loopback across 5 or 6 or hops. Of course, there are some providers
that have their BGP speaking routers not directly connected to a customer
router. You would then need to adjust the TTL accordingly (even if you are
not using loopbacks as the neighbor address)
-Eric
-- Eric Pylko epylko@frontrunnernetworks.com CCIE #5827 Mobile: (585) 747-2446 Systems Engineer Phone: (585) 899-4424 Frontrunner Network Systems Fax: (585) 899-4454-----Original Message----- From: cebuano [mailto:cebu2ccie@cox.net] Sent: Wednesday, October 16, 2002 11:18 PM To: ccielab@groupstudy.com Subject: RE: EBGP Multihop's necessity with loopback addresses
I need a clarification on the use of "ebgp-multihop" command. The only place so far that I've seen the TTL specified is in Doyle's Vol.2. Neither the CCO configuration guide nor Halabi has any examples showing the TTL actually being specified. Is this *necessary* or does this command dynamically adjust the value, sort of like traceroute? Thanks. Elmer
-----Original Message----- From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Peter van Oene Sent: Wednesday, October 16, 2002 9:11 PM To: Paglia, John (USPC.PCT.Hopewell); 'rem@digdomsol.com'; 'ccielab@groupstudy.com' Subject: RE: EBGP Multihop's necessity with loopback addresses
EBGP packets hit the wire with a TTL of 1 which pretty much makes them link local. If you want to peer over multiple hops, you need to change this TTL value which is done using the mulithop command.
At 07:43 PM 10/16/2002 -0400, Paglia, John (USPC.PCT.Hopewell) wrote: >Thanks for the reply. Actually, I am not having a problem. I was just >wondering about something someone told me. I never have configured 'ebgp-mu' >for connected neighbors, but this person said 'you need it if using >'update-source loop 0' because loop 0 isn't directly connected to the >neighbor.' > >I just wanted to know if anyone else has heard of such madness. I've never >configured BGP like this and it has never been a problem. > >Thanks, >John > > > -----Original Message----- > > From: Rem [SMTP:rem@digdomsol.com] > > Sent: Wednesday, October 16, 2002 7:27 PM > > To: Paglia, John (USPC.PCT.Hopewell) > > Subject: RE: EBGP Multihop's necessity with loopback addresses > > > > Are you running an igp between those neighbors? > > > > Where ebgp-multihop comes into play is between neighbors that are not > > directly connected and run only ebgp between them, ie your 2 loopback > > interfaces. if you run a trace between these 2 while running only ebgp you > > will see that they will not be able to perform a tcp connection, that is > > because the loopbacks do not know how to get to the remote side. if an igp > > is running between your serial connection then the route is established > > and > > the tcp connection can occur. > > > > try removing any internal routing protocol from your tables and see if it > > drops the connection. it has been my experience that this is currently the > > case and i know from experience that it has been a problem. watch out for > > it > > on the test when your start redistributing and the igp route goes away and > > all of a sudden your bgp drop also. its a nasty little thing if you don't > > see it coming. > > > > hth > > Ross > > > > -----Original Message----- > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of > > Paglia, John (USPC.PCT.Hopewell) > > Sent: Wednesday, October 16, 2002 1:01 PM > > To: 'ccielab@groupstudy.com' > > Subject: EBGP Multihop's necessity with loopback addresses > > > > > > I recently heard that if you are establishing your BGP neighbors using > > 'update source loopback 0', you should also use the 'ebgp-mu' cmd, even if > > the neighbors are directly connected...the reason being that your loopback > > is NOT directly connected to the neighbor. However, in my experiments I > > have > > never done this for neighbors that are directly connected, yet have > > established peerings successfully. > > > > Is there validity to this statement, and if so, under which circumstances > > is > > it absolutely vital, other than the 'non-physically or nbma topology' > > scenarios??? Something tells me that this may be an older IOS issue or > > something like that. > > > > John
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