I suppose the only other question is? Does this solution work for you. Regards, Joe Sanchez
( please excuse the brevity of this email as it was sent via a mobile device. Please excuse misspelled words or sentence structure.) On Dec 1, 2013, at 10:23 PM, Narendra Naukwal <[email protected]> wrote: > Marko, > > Here is an attempt to explain what BGP backdoor does. > > Topology: > > > > R1----EIGRP-----R3-----EBGP---R4 > | | > --EBGP--R2-EBGP > In this topology we need EIGRP link, between R1 & R3, to be primary as it is > a high Bandwidth link. > However, because EBGP's AD being 20, Route on R3 to prefix 1.1.1.1/32 is > preferred via R2.(EBGP’s AD 20 wins Over EIGRP's AD 90 for prefix 1.1.1.1/32) > > Checking Route on R3: > R3#sh ip route 1.1.1.1 > Routing entry for 1.1.1.1/32 > Known via "bgp 3", distance 20, metric 0 <<<< Known via BGP, distance - 20 > Tag 2, type external > Last update from 23.0.0.2 00:18:21 ago > Routing Descriptor Blocks: > * 23.0.0.2, from 23.0.0.2, 00:18:21 ago > Route metric is 0, traffic share count is 1 > AS Hops 2 > Route tag 2 > > So we see that route is being installed via BGP. Let s check the EIGRP > Topology table: > R3#sh ip eigrp topology 1.1.1.1/32 > IP-EIGRP (AS 1): Topology entry for 1.1.1.1/32 > State is Passive, Query origin flag is 1, 0 Successor(s), FD is 4294967295 > <<< FD - Infinite as a better route entry already exists in the RIB > Routing Descriptor Blocks: > 13.0.0.1 (FastEthernet1/0), from 13.0.0.1, Send flag is 0x0 > Composite metric is (156160/128256), Route is Internal > Vector metric: > Minimum bandwidth is 100000 Kbit > Total delay is 5100 microseconds > > Enabling debug for BGP, before configuring backdoor: (debug ip routing, debug > ip bgp, debug ip bgp update) > R3(config)#router bgp 3 > R3(config-router)#network 1.1.1.1 m 255.255.255.255 backdoor > > RT: del 1.1.1.1/32 via 23.0.0.2, bgp metric [20/0] <<< BGP AD = 20 & route > is being deleted > RT: delete subnet route to 1.1.1.1/32 > RT: NET-RED 1.1.1.1/32 > RT: delete network route to 1.0.0.0 > RT: NET-RED 1.0.0.0/8 > BGP(0): add request for 1.1.1.1/32 > BGP(0): Add request installing 1 of 1 routes for 1.1.1.1/32 -> > 23.0.0.2(main) to main IP table > RT: add 1.1.1.1/32 via 23.0.0.2, bgp metric [200/0] <<< Adding the route > with BGP AD = 200! > RT: NET-RED 1.1.1.1/32 > BGP(0): add request for 1.1.1.1/32 > BGP(0): Add request installing 1 of 1 routes for 1.1.1.1/32 -> > 23.0.0.2(main) to main IP table > BGP_Router: unhandled major event code 128, minor 0 > RT: closer admin distance for 1.1.1.1, flushing 1 routes <<< RIB realizes > that EIGRP path is now better than BGP's AD of 200. > RT: NET-RED 1.1.1.1/32 > BGP(0): lost route 1.1.1.1/32 for main IP table > RT: add 1.1.1.1/32 via 13.0.0.1, eigrp metric [90/156160] > RT: NET-RED 1.1.1.1/32 > > R3#sh ip route 1.1.1.1 > Routing entry for 1.1.1.1/32 > Known via "eigrp 1", distance 90, metric 156160, type internal > Redistributing via eigrp 1 > Last update from 13.0.0.1 on FastEthernet1/0, 00:08:25 ago > Routing Descriptor Blocks: > * 13.0.0.1, from 13.0.0.1, 00:08:25 ago, via FastEthernet1/0 > Route metric is 156160, traffic share count is 1 > Total delay is 5100 microseconds, minimum bandwidth is 100000 Kbit > Reliability 255/255, minimum MTU 1500 bytes > Loading 1/255, Hops 1 > > R3#sh ip bgp | inc 1.1.1.1 > r> 1.1.1.1/32 23.0.0.2 0 2 1 i > > Of course it shows RIB failure (due to Higher admin distance), but route > being best will still be advertised to any BGP neighbor of R3, R4 in our case. > > --> So in short what Backdoor does, is that it changes the AD of EBGP learnt > route to 200!! (AD for IBGP is in any case 200). > > Thanks, > Narendra Naukwal > > > > On Sunday, 1 December 2013 11:04 PM, Marko Milivojevic <[email protected]> > wrote: > > One of my favorites :-). What does it do ? :-) > > > On Sun, Dec 1, 2013 at 7:33 AM, Joe Sanchez <[email protected]> wrote: > Use the backdoor command for the particular network you want to be seen as > better from the igp. > > Regards, > Joe Sanchez > > ( please excuse the brevity of this email as it was sent via a mobile device. > Please excuse misspelled words or sentence structure.) > > On Nov 30, 2013, at 9:12 PM, Narendra Naukwal <[email protected]> wrote: > > > Hi Imran, > > > > Yes that is what I exactly meant. > > Distance in BGP is used for competing with other protocols only. In other > > routing protocols (like EIGRP) distance can be tweaked to choose internal > > over erxternal, however, that is not the case with BGP. > > External routes are always preferred over internal & this is not based on > > distance. > > Here is a detailed analysis of why router selected external path: > > > > > > 4#show ip bgp 1.1.1.0 > > BGP routing table entry for 1.1.1.0/24, version 4 > > Paths: (2 available, best #2, table Default-IP-Routing-Table) Advertised to > > update-groups: 1 12 10.0.0.1 from 20.0.0.3 (20.0.0.3) Origin > > IGP, metric 0, localpref 100, valid, internal 12 20.0.0.2 from 20.0.0.2 > > (20.0.0.2) Origin IGP, localpref 100, valid, external, best > > Comparing two paths as per the BGP path selection process. Comparing path 1 > > with path 2: Both paths have reachable next hops Both paths have a WEIGHT > > of 0 Both paths have a LOCAL_PREF of 100 Both paths are learned Both paths > > have AS_PATH length 1 Both paths are of origin IGP Both paths have the same > > neighbor AS, 12, so comparing MED. Both paths have a MED of 0 Path 2 is > > better than path 1 because an external path is preferred over an internal > > path. {Not because of distance but because the protocols developers wanted > > it to be this way :)} This is the reason that the best path is #2As per the > > BGP best path selection process, External is preferred if all the above > > parameters are same. > > > > Thanks, > > Narendra > > > > > > > > > > > > On Sunday, 1 December 2013 3:01 AM, Marko Milivojevic <[email protected]> > > wrote: > > > >> changing AD of ebgp to 201 forces ibgp routes...to get into the routing > >> table..... > > > > Imran - this statement is not correct. > > > > > > -- > > Marko Milivojevic - CCIE #18427 (SP R&S) > > Senior CCIE Instructor / Managing Partner - IPexpert > > :: Free Video Training: http://youtube.com/IPexpertInc > > :: Social: http://twitter.com/@icemarkom | http://fb.me/ccie18427 > > :: IPexpert: http://www.ipexpert.com/Communities | +1-810-326-1444 > > _______________________________________________ > > Free CCIE R&S, Collaboration, Data Center, Wireless & Security Videos :: > > > > iPexpert on YouTube: www.youtube.com/ipexpertinc > > > _______________________________________________ Free CCIE R&S, Collaboration, Data Center, Wireless & Security Videos :: iPexpert on YouTube: www.youtube.com/ipexpertinc
