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| 9.3 |  | Configuring BGP |
| 9.3.1 |  | Basic BGP configuration |

BGP configuration commands appear on the surface to mirror the syntax of familiar IGP commands. Examples of familiar IGP commands are RIP and OSPF. Although the syntax is similar, the function of these commands is significantly different.

To begin configuring a BGP process, issue the following familiar command:

> router(config)#router bgp AS-number

Notice that the Cisco IOS permits only one BGP process to run at a time. Therefore, a router cannot belong to more than one BGP AS.

The

network command is used with IGPs, such as RIP, to determine the interfaces on which to send and receive updates. The command also indicates which directly connected networks to advertise. However, when configuring BGP, the network command does not affect what interfaces BGP runs on. Therefore, configuring just a network statement will not establish a BGP neighbor relationship. This is a major difference between BGP and IGPs. The network statement follows this syntax:

> Router(config-router)#network network-number [mask network-mask]

In BGP, the

network command tells the BGP process what locally learned networks to advertise. The networks can be connected routes, static routes, or routes learned by way of a dynamic routing protocol, such as RIP. These networks must also exist in the routing table of the local router or they will not be sent out in updates. The mask keyword can be used with the network command to specify individual subnets. Routes learned by the BGP process are propagated by default but are often filtered by a routing policy.

For a BGP router to establish a neighbor relationship with another BGP router, issue the following configuration command:

> Router(config-router)#neighbor ip-address remote-as AS-number

This command serves to identify a peer router with which the local router will establish a session. The

ip-address argument is the IP address of the neighbor interface. The AS-number argument determines whether the neighbor router is an EBGP or an IBGP neighbor.

| Lab Activity e-Lab Activity: Basic BGP Configuration This lab is to configure a basic BGP configuration.

| Web Links Configuring BGP http://www.cisco.com/univercd/cc/td/doc/product/ software/ios122/122cgcr/fipr_c/ipcprt2/1cfbgp.htm