This post represents the solution and explanation for quiz-4.
Have a look at it to understand the problem.
As described in the quiz, you try to establish a BGP external peering between the border routers of two offices. These routers can reach each using the default route that each of them has via the ISP.
sh ip cef 126.96.36.199 0.0.0.0/0, version 9, epoch 0, cached adjacency to Serial0/0 0 packets, 0 bytes via 188.8.131.52, 0 dependencies, recursive next hop 184.108.40.206, Serial0/0 via 220.127.116.11/30 valid cached adjacency
[UPDATE] According to the Cisco documentation this is done in order to avoid route flapping and routing loops.
In this situation,
deb ip bgp eventBGP events debugging is on Office-1# *Mar 1 00:12:39.931: BGP: Import timer expired. Walking from 1 to 1 *Mar 1 00:12:54.931: BGP: Import timer expired. Walking from 1 to 1 Office-1# Office-1#deb ip bgp ipv4 unicast BGP debugging is on for address family: IPv4 Unicast Office-1# *Mar 1 00:31:36.603: BGP: 18.104.22.168 active open failed - no route to peer, open active delayed 31267ms (35000ms max, 28% jitter) *Mar 1 00:32:07.871: BGP: 22.214.171.124 active open failed - no route to peer, open active delayed 32532ms (35000ms max, 28% jitter) Office-1#
Let's configure a static route on Office-1 router - soon after that the BGP session goes up:
ip route 126.96.36.199 255.255.255.255 188.8.131.52Office-1(config)#end Office-1# Office-1# *Mar 1 00:22:34.607: %BGP-5-ADJCHANGE: neighbor 184.108.40.206 UpOffice-1# Office-1#sh ip bgp s BGP router identifier 220.127.116.11, local AS number 65100 BGP table version is 1, main routing table version 1 Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 18.104.22.168 4 65200 4 4 1 0 0 00:00:17 0 Office-1# Office-1# Office-1#sh ip bgp nei 22.214.171.124 | inc state|host BGP state = Established, up for 00:00:41 Connection state is ESTAB, I/O status: 1, unread input bytes: 0 Local host: 126.96.36.199, Local port: 53209Foreign host: 188.8.131.52, Foreign port: 179 Office-1# Office-1#sh tcp brief TCB Local Address Foreign Address (state) 6752A090 184.108.40.206.53209 220.127.116.11.179ESTAB Office-1#
As you can see in both outputs
sh ip bgp nei and
sh tcp brief, Office-1 router initiates the TCP connection (
Another interesting thing is that the route to the peer can as well be learned via BGP. There were people that replied in the quiz saying that "(quote) the route to the peer must not be default nor learned via BGP (endofqoute)" - the 2nd part is not true ! Here is an output showing that the BGP is established even though the route to the BGP peer is learned via another BGP session:
sh ip bgp sum... Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 18.104.22.168 4 65100 10 10 2 0 0 00:05:50 022.214.171.124 4 100 11 11 2 0 0 00:07:00 1 Office-2# Office-2# sh ip route 126.96.36.199Routing entry for 188.8.131.52/32 Known via "bgp 65200", distance 20, metric 0 ... Office-2# Office-2# sh tcp brTCB Local Address Foreign Address (state) 650B7CB8 184.108.40.206.55415 220.127.116.11.179ESTAB
In this output, Office-2 router has an eBGP peering with the ISP and learns the IP of Office-1 (18.104.22.168) via this peering - which is enough to start a TCP connection to Office-1.
- a BGP speaker
will not initiatethe TCP session to establish a BGP peering if the peer is reachable only over a default route.
You'll need a more specific route than default - it can br learned statically or dynamically (including via another BGP session - it's still ok)
- a BGP speaker
will accept/respondto a TCP session and will establish a BGP peering even if the peer is reachable over a default route. Thus, the non-default route is only needed on one side, but it's always recommended to exist on both sides
- once established, the BGP peering will not be broken if the more specific route is lost and connectivity remains over the default route
Thanks everyone for your comments in the quiz !