EBGP and IBGP Enabling BGP routing The command used to enable BGP on a router is: router bgp autonomous-system RTA# router bgp 100 RTB# router bgp 200 The above statements indicate that RTA is running BGP and it belongs to AS100 and RTB is running BGP and it belongs to AS200 and so on. BGP Neighbors/Peers The neighbor command used to establish a TCP connection is: neighbor ip-address remote-as number The remote-as number is the AS number of the router we are trying to connect to via BGP. The ip-address is the next hop directly connected address for EBGP1 and any IP address2 on the other router for IBGP. clear ip bgp address (where address is the neighbor address) clear ip bgp * (clear all neighbor connections)
By default, BGP sessions begin using BGP Version 4 and negotiating downward to earlier versions if necessary. To PRevent negotiations and force the BGP version used to communicate with a neighbor, perform the following task in router configuration mode: neighbor { ip address peer-group-name} version value An example of the neighbor command configuration follows: RTA# router bgp 100 neighbor 129.213.1.1 remote-as 200 RTB# router bgp 200 neighbor 129.213.1.2 remote-as 100 neighbor 175.220.1.2 remote-as 200 RTC# router bgp 200 neighbor 175.220.212.1 remote-as 200 BGP and Loopback interfaces Using a loopback interface to define neighbors is commonly used with IBGP rather than EBGP. Normally the loopback interface is used to make sure that the IP address of the neighbor stays up and is independent of an interface that might be flaky. In the case of EBGP, most of the time the peer routers are directly connected and loopback does not apply. If the IP address of a loopback interface is used in the neighbor command, some extra configuration needs to be done on the neighbor router. The neighbor router needs to tell BGP that it is using a loopback interface rather than a physical interface to initiate the BGP neighbor TCP connection. The command used to indicate a loopback interface is: neighbor ip-address update-source interface The following example should illustrate the use of this command. RTA# router bgp 100 neighbor 190.225.11.1 remote-as 100 neighbor 190.225.11.1 update-source loopback 1 RTB# router bgp 100 neighbor 150.212.1.1 remote-as 100 RTA# router bgp 100 neighbor 180.225.11.1 remote-as 300 neighbor 180.225.11.1 ebgp-multihop RTB# router bgp 300 neighbor 129.213.1.2 remote-as 100
Enabling BGP routing 
