eBGP Disable Connected Check

Using disable-connected-check is a convenient way to peer two directly connected routers without utilizing ebgp-multihop. Make sure you understand that the bgp neighborship will not be attempted if it is not connected directly.

If we do not configure ebgp-multihop to modify the default TTL value, then the BGP messages are delivered with a TTL of 1.A router that has enabled ebgp-multihop will automatically and implicitly treat the neighbors as if the disable-connected-check feature is configured. To put it another way, using ebgp-multihop automatically disables connected-check

The disable-connected-check option is ignored when we use either TTL-security or eBGP multihop with a numerical argument of 2 or more hops. Whenever it has been activated, it does not need to be configured.

Fig 1.1- Disable connected check in BGP

Configurations on NDNA_R1

router bgp 100
neighbor 10.10.10.2 remote-as 200
neighbor 10.10.10.2 update-source loopback 0
neighbor 10.10.10.2 disable-connected-check
!
ip route 10.10.10.2 255.255.255.255 s0/0
ip route 10.10.10.2 255.255.255.255 s1/0
!

NDNA_R1 # sh ip bgp summary
BGP router identifier 5.5.5.1, local AS number 100
Neighbor    V   AS   MsgRcvd   MsgSent   TblVer  InQ   OutQ   Up/Down     State 
10.10.10.2  4   200   468       510       266     0     0      07:26:43    up           Established
20.20.20.2  4   300   162       221       134     0     0      01:12:13     up          Established