Yes, it is no brainer. My point will be expressed much cleaner if I will add that second route is learned via iBGP, and first via eBGP.
Please see: r1#sh ip bgp summ BGP router identifier 10.10.1.1, local AS number 100 BGP table version is 2, main routing table version 2 1 network entries and 1 paths using 133 bytes of memory 1 BGP path attribute entries using 60 bytes of memory 1 BGP AS-PATH entries using 24 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory BGP activity 1/8 prefixes, 2/1 paths, scan interval 60 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.1.1.8 4 1 55 55 2 0 0 00:51:29 1 10.10.1.2 4 100 54 53 2 0 0 00:48:57 0 r1# telnet-server#2 [Resuming connection 2 to r2 ... ] r2#sh ip bgp summ BGP router identifier 172.168.32.1, local AS number 100 BGP table version is 4, main routing table version 4 1 network entries and 2 paths using 169 bytes of memory 2 BGP path attribute entries using 120 bytes of memory 2 BGP AS-PATH entries using 48 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory BGP activity 1/13 prefixes, 2/0 paths, scan interval 60 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.1.1.6 4 2 48 49 4 0 0 00:42:30 1 10.10.1.1 4 100 54 55 4 0 0 00:49:08 1 r2# Please also note that r1 is not learning from r2 via iBGP any routes, because r2 is not advertising any routes. r2#sh ip bgp neighbors 10.10.1.1 advertised-routes r2# And the reason is that active (selected) bgp route on r2 is learned from r1 via iBGP so it cannot be advertised back. Przemek Ps. Sorry for line wraps, I just switched to Evolution and dont know yet how to increase line length :-) On Tue, 2002-02-05 at 20:44, W. Alan Robertson wrote: > > >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" > To: "Peter van Oene" > Cc: "W. Alan Robertson" ; "Groupstudy - > CCIELAB" ; "Groupstudy - Cisco Certification" > > 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 > > > > > > _________________________________________________________________ > > > > > CCIE Security list: > http://www.groupstudy.com/list/security.html > > > >_________________________________________________________________ > > > >CCIE Security list: http://www.groupstudy.com/list/security.html Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=34559&t=34559 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
