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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
//关于BGP and loopback interfaces中的配置实例中：
RTA#
router bgp 100
neighbor 190.225.11.1 remote-as 100
neighbor 190.225.11.1 update-source int loopback 1
应是：
neighbor 190.225.11.1 update-source loopback 1