3.7.b Implement and troubleshoot IBGP and EBGP

3.7.b [i] EBGP, IBGP

the conventional wisdom for directly connected EBGP routers when using loopbacks as the update source is to manually set the ebgp-multihop to a number greater than 1 in order to establish the peering… this manually sets the ttl value  as seen below to a number other than the default…

bgp_ttl_mhop

the disable-connected-check accomplishes the same thing without changing the ttl value…

bgp_disable_connect_cap

according to cisco here: http://www.cisco.com/en/US/docs/ios/iproute_bgp/command/reference/irg_bgp3.html#wp1106122

disable connected check

A BGP routing process will verify the connection of single-hop eBGP peering session (TTL=254) to determine if the eBGP peer is directly connected to the same network segment by default. If the peer is not directly connected to same network segment, connection verification will prevent the peering session from being established.

neighbor ebgp-multihop

To accept and attempt BGP connections to external peers residing on networks that are not directly connected, use the neighbor ebgp-multihop command in router configuration mode. To return to the default, use the no form of this command.

they accomplish the same thing…

this guy here has more on it:

http://lostintransit.se/tag/disable-connected-check/