From: Peter van Oene (pvo@xxxxxxxxxxxx)
Date: Wed Feb 06 2002 - 12:19:23 GMT-3
I was assuming this was a choice over similar paths..
At 08:27 PM 2/5/2002 -0500, Przemyslaw Karwasiecki wrote:
>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
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