The disable-connected-check was created precisely for the purpose of peering two directly connected routers on their loopbacks without using the ebgp-multihop.
BGP Disable-Connected-Check
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
Related- Troubleshooting indirectly connected eBGP neighbors
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 –
Example Scenario
The devices R1 and R2 are arranged as per the above-shown diagram.
Configuration
R1:
interface Loopback0
ip address 1.1.1.1 255.255.255.255
!
interface FastEthernet0/0
ip address 9.9.12.1 255.255.255.0
!
router bgp 100
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 200
neighbor 2.2.2.2 disable-connected-check
neighbor 2.2.2.2 update-source Loopback0
!
ip route 2.2.2.2 255.255.255.255 9.9.12.2
R2:
interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface FastEthernet0/0
ip address 9.9.12.2 255.255.255.0
!
router bgp 200
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 disable-connected-check
neighbor 1.1.1.1 update-source Loopback0
!
ip route 1.1.1.1 255.255.255.255 9.9.12.1
Verification
R1#sh ip bgp summary
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
2.2.2.2 4 200 7 7 1 0 0 00:02:57 0
R1#
As shown above , neighborship over Loopbacks is being formed without need of “eBGP Multihop” configuration requirement .
Related- BGP Multipath Scenario
ABOUT THE AUTHOR
I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”
I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.
I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)
