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 > > > > _________________________________________________________________ > > > > CCIE Security list: http://www.groupstudy.com/list/security.html > > >_________________________________________________________________ > > >CCIE Security list: http://www.groupstudy.com/list/security.html
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