BGP学习精华

　　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

　　//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