The disable-connected-check was created precisely for the purpose of peering two directly connected routers on their loopbacks without using the ebgp-multihop.
By default there are two precautions when establishing eBGP neighborships:
- Check whether the client is on a directly connected network. If it is not, there won’t be any attempt to form the neighbourship. This check can be deactivated on a per-neighbor basis using the neighbor disable-connected-check
- The default TTLvalue for the packets carrying the BGP messages is 1, unless we configure ebgp-multihop to modify the TTLvalue. If the ebgp-multihop is configured, the router automatically and implicitly behaves to the neighbor as if the disable-connected-check was configured. In other words, using ebgp-multihop automatically implies disable-connected-check
Disable-connected-check is not used when we use either ttl-security or eBGP multihop with a numerical argument of 2 or greater hops. In these cases disable-connected-check automatically gets activated and doesn’t require specific configurations.Disable-connected-check is configured under router BGP process along with neighbor statement.
Below scenario will help understand the concept of “Disable-Connected-Check” in more detailed way –
The devices R1 and R2 are arranged as per above shown diagram.
R1#sh ip bgp summary
As shown above , neighborship over Loopbacks is being formed without need of “eBGP Multihop” configuration requirement .