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 neighborship. This check can be deactivated on a per-neighbor basis using the neighbor disable-connected-check
- The default TTL value for the packets carrying the BGP messages is 1 unless we configure ebgp-multihop to modify the TTL value. 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, it automatically gets activated and doesn’t require specific configurations. This is configured under the router BGP process along with the neighbor statement.
Related- TTL Security in BGP
Below scenario will help understand the concept of “Disable-Connected-Check” in a more detailed way –
The devices R1 and R2 are arranged as per the above-shown diagram.
R1#sh ip bgp summary
As shown above , neighborship over Loopbacks is being formed without need of “eBGP Multihop” configuration requirement .
Related- BGP Multipath Scenario