USING “DISABLE-CONNECTED-CHECK” IN CISCO BGP

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:

  1. Check whether the client is on a directly connected network. If it is not, there won’t be any attempt to form the neighbourship. This check can be deactivated on a per-neighbor basis using the neighbor disable-connected-check
  2. The default TTLvalue for the packets carrying the BGP messages is 1, unless we configure ebgp-multihop to modify the TTLvalue. 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 disable-connected-check automatically gets activated and doesn’t require specific configurations.Disable-connected-check is configured under router BGP process along with neighbor statement.

Below scenario will help understand the concept of “Disable-Connected-Check” in more detailed way –

 

EXAMPLE SCENARIO

 

The devices R1 and R2 are arranged as per 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 router identifier 1.1.1.1, local AS number 100

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 .

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