BGP Trick: Peering Double Default Route

Hi mates,

I am just writing to put here a trick that I have found out after doing
some testing in BGP peering. Let4s say we have two routers (R1&R2) directly
connected and they're going to establish an eBGP session between their
loopbacks.They will use update-source with the loopback and
disable-connected check. However, the reachability is via a default route
in each router, therefore, the BGP session can not be established:

BGP: *2.2.2.2 active open failed - no route to peer*, open active delayed
32091ms (35000ms max, 28% jitter)

What I thought was, how could they go ahead and establish the session
without changing the routing? What if I install a route to the loopback
pointing to null 0 to bypass the BGP check and then, remove it ?
I tested it and surprisingly it worked! Look at that:

*R1(config)#ip route 2.2.2.2 255.255.255.255 null 0
*R1(config)#
*Mar 1 00:05:11.751: BGP: 2.2.2.2 open active, local address 1.1.1.1
*R1(config)#no ip route 2.2.2.2 255.255.255.255 null 0
*R1(config)#
*Mar 1 00:05:17.843: BGP: 2.2.2.2 went from Active to OpenSent
*Mar 1 00:05:17.847: BGP: 2.2.2.2 sending OPEN, version 4, my as: 100,
holdtime 180 seconds
*Mar 1 00:05:17.851: BGP: 2.2.2.2 send message type 1, length (incl.
header) 45
*Mar 1 00:05:17.903: BGP: 2.2.2.2 rcv message type 1, length (excl.
header) 26
*Mar 1 00:05:17.907: BGP: 2.2.2.2 rcv OPEN, version 4, holdtime 180 seconds
*Mar 1 00:05:17.907: BGP: 2.2.2.2 rcv OPEN w/ OPTION parameter len: 16
*Mar 1 00:05:17.907: BGP: 2.2.2.2 rcvd OPEN w/ optional parameter type 2
(Capability) len 6
*Mar 1 00:05:17.907: BGP: 2.2.2.2 OPEN has CAPABILITY code: 1, length 4
*Mar 1 00:05:17.907: BGP: 2.2.2.2 OPEN has MP_EXT CAP for afi/safi: 1/1
*Mar 1 00:05:17.911: BGP: 2.2.2.2 rcvd OPEN w/ optional parameter type 2
(Capability) len 2
*Mar 1 00:05:17.911: BGP: 2.2.2.2 OPEN has CAPABILITY code: 128, length 0
*Mar 1 00:05:17.911: BGP: 2.2.2.2 OPEN has ROUTE-REFRESH capability(old)
for all address-families
*Mar 1 00:05:17.911: BGP: 2.2.2.2 rcvd OPEN w/ optional parameter type 2
(Capability) len 2
*Mar 1 00:05:17.911: BGP: 2.2.2.2 OPEN has CAPABILITY code: 2, length 0
*Mar 1 00:05:17.915: BGP: 2.2.2.2 OPEN has ROUTE-REFRESH capability(new)
for all address-families
BGP: 2.2.2.2 rcvd OPEN w/ remote AS 200
*Mar 1 00:05:17.915: BGP: 2.2.2.2 went from OpenSent to OpenConfirm
*Mar 1 00:05:17.915: BGP: 2.2.2.2 went from OpenConfirm to Established
*Mar 1 00:05:17.915: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Up

Just for fun!

Blogs and organic groups at http://www.ccie.net
Received on Wed Jul 18 2012 - 13:44:05 ART