[117714] in North American Network Operators' Group
RE: bgp best path compare-routerid implementation example
daemon@ATHENA.MIT.EDU (Austin Wilson)
Fri Sep 25 13:51:42 2009
From: Austin Wilson <austinw@bandcon.com>
To: devang patel <devangnp@gmail.com>, "nanog@nanog.org" <nanog@nanog.org>
Date: Fri, 25 Sep 2009 10:50:44 -0700
In-Reply-To: <d0fea3580909242124m4d8d878y8bcac2e3c583467c@mail.gmail.com>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
Dev,
This is usually used to offset the oldest route metric. The problem is that=
when a link fails and comes back online, traffic can shift from one provid=
er to another in the middle of your billing cycle. This then could mean you=
get double billed for that traffic. People use the command to basically tu=
rn off the oldest route metric and use the routerid (not peering ip) to mak=
e the last decision on where to send your traffic. This is commonly called =
"tie breaker" traffic. If a peer fails with this command enabled, when the =
peer comes back online, traffic should be restored to the original level be=
fore the failure.=20
A possible issue with this command is that if a local peer's route/session =
flaps it could have more of an effect on your network/router as it will alw=
ays try move those routes back to the FIB. That's probably why they impleme=
nted this metric in the first place, the oldest route is the most stable. A=
nother issue is that you are at the mercy of vendor's routerid when your ro=
uter decides where to send your "tie breaker" traffic. Level3 gets most of =
this traffic since they have such low routeids.
There are ways to get around this problem and take back control of your tie=
breaker traffic. Dani did a pretty good tutorial on this issue and its loc=
ated here:
http://nanog.org/meetings/nanog46/abstracts.php?pt=3DMTM3MiZuYW5vZzQ2&nm=3D=
nanog46
Basically he suggests using MEDs to change the tie breaker as part of a com=
plete BGP traffic engineering solution. Doing the things listed there and e=
lsewhere will mean you won't even have to use this command.=20
Austin Wilson
-----Original Message-----
From: devang patel [mailto:devangnp@gmail.com]=20
Sent: Thursday, September 24, 2009 9:24 PM
To: nanog@nanog.org
Subject: bgp best path compare-routerid implementation example
Hello Nanog,
I am looking for the *bgp best path compare*-*routerid* implementation
example? I know the function of it but looking for some scenario where its
been used...
Thanks,
Dev
