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.

 

Diagram:

update_source

 

Configuration and verification:

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

R10(config-router)#neighbor 10.100.1.6 remote-as 100
R10(config-router)#neighbor 10.100.1.6 update-source lo0

R6(config-router)#neighbor 10.10.1.10 remote-as 10
R6(config-router)#neighbor 10.10.1.10 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 10.100.1.6 SO lo0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.100.1.6, timeout is 2 seconds:
Packet sent with a source address of 10.10.1.10
.....
Success rate is 0 percent (0/5)

R10(config)#ip route 10.100.1.6 255.255.255.255 eth0/0

R6(config)#ip route 10.10.1.10 255.255.255.255 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 10.100.1.6 disable-connected-check

R6(config-router)#neighbor 10.10.1.10 disable-connected-check

 

Now the BGP session is UP.

*Jul  7 20:35:00.053: %BGP-5-ADJCHANGE: neighbor 10.100.1.6 Up

R10#sh ip bgp neigh 10.100.1.6
BGP neighbor is 10.100.1.6,  remote AS 100, external link
  BGP version 4, remote router ID 10.100.1.6
  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)
SNIP
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