BGP TCP Session Initiation Control

It is possible to control which router initiates a BGP session request and which one passively waits to hear from the other.

R1(config)#router bgp 100
R1(config-router)#neighbor 192.168.12.2 transport ?
  connection-mode     Specify passive or active connection
  multi-session       Use Multi-session for transport
  path-mtu-discovery  Use transport path MTU discovery

For a simple topology with two routers R1 & R2, "transport connection-mode" can be configured to make R1 active and R2 passive.

R1
router bgp 100
 neighbor 192.168.12.2 remote-as 100
 neighbor 192.168.12.2 transport connection-mode active

R2 waits passively to hear from the peer.

R2
router bgp 100
 neighbor 192.168.12.1 remote-as 100
 neighbor 192.168.12.1 transport connection-mode passive

R1 acts as the TCP client and R2 as the server listening on TCP port 179.

To validate the session establishment:

R1#show tcp brief
TCB       Local Address               Foreign Address             (state)
11029800  192.168.12.1.47867           192.168.12.2.179              ESTAB

The "transport multi-session" option enables BGP capability to form multiple TCP sessions between neighbors where there is only one neighbor statement. It is relevant when Multi Topology Routing (MTR) is a required use-case.

BGP TCP Session Initiation Control

Further Reading