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
