IP Routing

Getting BGP Running

Starting BGP on a router is similar to starting any other routing process, such as RIP or IGRP. The command to start BGP is:

router bgp AutonomousSystemNumber

where AutonomousSystemNumber is the AS number of the local router.

This is where the similarity with other routing protocols stops. When configured under BGP, the following network statement:

network IPAddress [mask A.B.C.D]

specifies the prefix to announce to BGP peers. Compare this with the configuration of IGPs, where the network-number statement has very different semantics: it specifies the attached networks on which to discover neighbors or peers.

Speaking of peers, there are no mechanisms in BGP-4 to automatically discover neighbors. BGP-4 requires that peers must be specified by IP address. The command to specify a peer is:

neighbor IPAddress remote-as AutonomousSystemNumber

where IPAddress specifies the peer with an AS number of AutonomousSystemNumber.

Let’s look at TraderMary’s configuration for its connection to ISP-A, as shown in Figure 7-3. Line 1 in the following code block starts BGP with a local AS number of 100. Line 3 specifies that the prefix 192.200.200.0/26 be announced to TrdrMary-1’s BGP peers. Line 4 specifies that the network number 30.0.0.0 be announced as well, with an 8-bit mask (the natural classful mask is used when a mask is not specified). Line 5 specifies that all static routes should also be announced. (There are two static routes known to TrdrMary-1, as shown in lines 7 and 8). ...