Categories R&S

BGP – implement and troubleshoot Peerings – Neighbor Disable-Connected-Check

Following my previous post with ebgp multihop, we have another way to establish eBGP peering when neighbor are peering using their loopbacks.


Requirements :

Establish the eBGP session between R10 and R6 using their loopbacks.





Configuration and verification:

I configure the peering between R10 and R6 using their loopback.

R10(config-router)#neighbor remote-as 100
R10(config-router)#neighbor update-source lo0

R6(config-router)#neighbor remote-as 10
R6(config-router)#neighbor update-source lo0


The eBGP session is not coming UP.


First, we have no connectivity between our loopbacks, we will need to add a static route because the loopback are not seen as directly connected.

R10#PING SO lo0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Packet sent with a source address of
Success rate is 0 percent (0/5)

R10(config)#ip route eth0/0

R6(config)#ip route Ethernet0/1


Now the eBGP session is still not up.

Remember that by default the TTL for eBGP is set to one, because we are peering with the loopback we need to change that otherwisw the peering will not be establish.


We could have used the ebgp multihop command but when the two routers are directly connected, it is better to use the disable connected check command.


R10(config-router)#neighbor disable-connected-check

R6(config-router)#neighbor disable-connected-check


Now the BGP session is UP.

*Jul  7 20:35:00.053: %BGP-5-ADJCHANGE: neighbor Up

R10#sh ip bgp neigh
BGP neighbor is,  remote AS 100, external link
  BGP version 4, remote router ID
  BGP state = Established, up for 00:02:25
  Last read 00:00:35, last write 00:00:35, hold time is 180, keepalive interval is 60 seconds
  Neighbor sessions:
    1 active, is not multisession capable (disabled)
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 1


As you can see the outgoing TTL is still set to 1.



Coming next, more BGP option for peering.




Thank you for reading.



BGP – implement and troubleshoot Peerings – Neighbor Disable-Connected-Check