From: W. Alan Robertson (warobertson@xxxxxxxxxxxxx)
Date: Tue Feb 05 2002 - 22:44:34 GMT-3
>From your 'sh ip bgp' output, it's a no-brainer that it selected the
second route... In addition to a Local Preference, you've got AS
pre-pending occurring on the path learned via 10.1.1.6. These two
routes are not "equal" in the eyes of BGP... One is a single AS hop
away, and the other is Four (4) AS Hops away.
You've also originated a route prefix in two separate AS's, which
while technically possible (I guess), is never supposed to happen.
Alan
----- Original Message -----
From: "Przemyslaw Karwasiecki" <karwas@ifxcorp.com>
To: "Peter van Oene" <pvo@usermail.com>
Cc: "W. Alan Robertson" <warobertson@earthlink.net>; "Groupstudy -
CCIELAB" <ccielab@groupstudy.com>; "Groupstudy - Cisco Certification"
<cisco@groupstudy.com>
Sent: Tuesday, February 05, 2002 8:27 PM
Subject: Re: Undocumented iBGP Behavior (Confirmed by Cisco)
> After siple lab experiment I need to disagree with your statement.
>
> > cisco by default prefers ebgp over ibgp. it should not, by
default, enjoy
> > the ibgp routes learned from the peer over the ebgp learned
routes.
>
> I belive that you are overinterpreting meaning of administrative
> distance.
>
> You are right that aministrative distance of eBGP routes is 20
> versus 200 for iBGP routes, but in the situation when BGP process
> receives 2 routes for the same prefix, it applies first standart
> BGP selection mechanism:
> http://www.cisco.com/warp/public/459/25.shtml
> and after best route is selected it is going to be inserted into
> routing table with specific administrative distance.
>
> I have replicated following scenario in my lab.
>
> There are 2 external ASes 1, and 2, originating
> prefix 1.1.1.0/24 and advertising it to 2 routers
> r1 and r2 via eBGP.
>
> Routers r1 and r2 are iBGP peers.
>
> Prefix 1.1.1.0/24 originated from AS2 has longer AS_PATH
> (as prepend applied 3 times)
>
>
> Please see folowing commands executed on r2:
>
> r2#sh ip bgp
> BGP table version is 4, local router ID is 172.168.32.1
> Status codes: s suppressed, d damped, h history, * valid, > best,
i -
> internal
> Origin codes: i - IGP, e - EGP, ? - incomplete
>
> Network Next Hop Metric LocPrf Weight Path
> * 1.1.1.0/24 10.1.1.6 0 0 2 2 2 2
i
> *>i 10.1.1.8 0 100 0 1 i
> r2#sh ip rou
> r2#sh ip route
> Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B -
BGP
> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter
area
> N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type
2
> E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
> i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS
> inter area
> * - candidate default, U - per-user static route, o - ODR
> P - periodic downloaded static route
>
> Gateway of last resort is not set
>
> 1.0.0.0/24 is subnetted, 1 subnets
> B 1.1.1.0 [200/0] via 10.1.1.8, 00:09:26
> 172.168.0.0/24 is subnetted, 1 subnets
> C 172.168.32.0 is directly connected, Loopback0
> 10.0.0.0/16 is subnetted, 2 subnets
> C 10.10.0.0 is directly connected, Serial0
> C 10.1.0.0 is directly connected, Ethernet0
> r2#
>
> As you can see, BGP process on r2 selects route learned
> from its iBGP peer over route learned via eBGP,
> and this route is eventualy inserted to routing table
> with administrative distance of 200
>
>
> Correct me if I am ovrlooking something,
> and thank you for excelent idea for testing.
>
>
> Przemek
>
>
> On Tue, 2002-02-05 at 19:35, Peter van Oene wrote:
> > cisco by default prefers ebgp over ibgp. it should not, by
default, enjoy
> > the ibgp routes learned from the peer over the ebgp learned
routes.
> >
> >
> >
> > At 05:37 PM 2/5/2002 -0500, Przemyslaw Karwasiecki wrote:
> > >Correct me if I am wrong but this:
> > >
> > > > if an iBGP peer learns that another iBGP peer already has a
better
> > > > route to a specific prefix, it will issue a withdrawl to that
peer
> > > > for the prefix(es).
> > >
> > >is perfectly normal, standart behaviour.
> > >If your Genuity route is better, you will select this route
> > >in your routing table, and if by any chance before you had
> > >there UUNET route which you have advertised, you need to send
> > >update with new, better, selected route.
> > >
> > >BGP will never advertise both routes.
> > >This is distant vector after all.
> > >
> > >So if during convergence phase your route selection
> > >is shuffling your routes in your Loc-RIB, you should
> > >to expect series of updates to follow up.
> > >
> > >Przemek
> > >
> > >
> > >On Tue, 2002-02-05 at 16:45, W. Alan Robertson wrote:
> > > > Folks,
> > > >
> > > > Just to let you know, I ran across what looked like a bug in
Cisco's
> > > > BGP code... Turns out, this is undocumented new behavior.
> > > >
> > > > We just deployed a pair of 3640s for one of our customers, for
> > > > dual-router, dual-homed Internet connectivity. We are taking
full
> > > > tables from Genuity (AS 1), and Worldcom (AS 701).
> > > >
> > > > Each router was learning 104,000+ prefixes from each of the
external
> > > > peers, but the iBGP peering was acting really strange. One of
the
> > > > routers was learning the full table from the other, but the
second
> > > > router was only taking like 700 prefixes.
> > > >
> > > > When we cleared the internal peer (soft or hard), we could see
the
> > > > whole table being transferred... It would climb as though it
were
> > > > going to learn them all, and then as it approached 100,000
prefixes,
> > > > it would rapidly drop back down to 700. I debugged the iBGP
peer, and
> > > > saw it issuing withdrawls for all of these routes.
> > > >
> > > > We opened a ticket with the TAC, and they initially believed
it to be
> > > > a bug as well. Upon further review, they came back and told
us that
> > > > this was the desired behavior in the newer code (We are
running
> > > > 12.0(20) on these boxes). In order to conserve memory, and
processor,
> > > > if an iBGP peer learns that another iBGP peer already has a
better
> > > > route to a specific prefix, it will issue a withdrawl to that
peer
> > > > for the prefix(es).
> > > >
> > > > I spent quite a while second guessing what seemed to be a very
simple,
> > > > straighforward configuration. I have done several near
identical
> > > > deployments in the past.
> > > >
> > > > I guess the moral is this: If you know your config is
correct, and
> > > > the router behavior is not what you expect, do not hesitate to
call
> > > > the TAC.
> > > >
> > > > I hope they are as helpful on Monday, when I call them from
the CCIE
> > > > Lab in RTP. ;)
> > > >
> > > > Regards...
> > > >
> > > > Alan
> > > >
This archive was generated by hypermail 2.1.4 : Thu Jun 20 2002 - 13:46:12 GMT-3