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). Line 6 specifies that TrdrMary-1’s BGP peer (ISP-A) has an IP address of 192.100.100.254 and an AS number of 192. This is the only neighbor statement, so in this example TrdrMary-1 has only one peer: ISP-A.

Figure 7-3. TraderMary’s connection to ISP-A

Here’s what the configuration looks like:

   hostname TrdrMary-1
   !
   interface Loopback0
    ip address 192.168.1.10 255.255.255.255
   !
   interface Ethernet0
    description * External Network *
    ip address 192.200.200.1 255.255.255.192
   !
   interface Ethernet1
    ip address 172.16.1.3 255.255.255.0
   !
   interface Serial1
    description * to ISP-A *
    ip address 192.100.100.253 255.255.255.252
   ...
1  router bgp 100  
2   no synchronization 
3   network 192.200.200.0 mask 255.255.255.192 
4   network 30.0.0.0 
5   redistribute static  
6   neighbor 192.100.100.254 remote-as 192         
   !
   ip classless
7  ip route 160.160.1.0 255.255.255.0 Ethernet1    
8  ip route 192.168.3.0 255.255.255.0 Ethernet1

The configuration on ISP-A-1 is very similar. Line 9 starts BGP on ISP-A-1 with the AS number 192. Line 10 specifies the default route being announced to TraderMary. Line 11 specifies that ISP-A-1’s BGP peer (TrdrMary-1) has an IP address of 192.100.100.253 and an AS number of 100:

    hostname ISP-A-1
    !
    interface Loopback9
     ip address 98.2.0.1 255.255.0.0
    !
    interface Serial0
     ip address 192.100.100.254 255.255.255.252
    !
9   router bgp 192
10   network 0.0.0.0
11   neighbor 192.100.100.253 remote-as 100
    !
    no ip classless
12  ip route 0.0.0.0 0.0.0.0 Null0

The first question to ask after both peers have been configured is whether the peers see each other. The following command checks the status of the neighbor relationship between TrdrMary-1 and ISP-A-1:

    TrdrMary-1#sh ip bgp neighbor 192.100.100.254
13  BGP neighbor is 192.100.100.254,  remote AS 192, external link
     Index 1, Offset 0, Mask 0x2
14    BGP version 4, remote router ID 98.2.0.1
15    BGP state = Established, table version = 5, up for 00:00:42
   ...

Line 15 indicates that the BGP state is Established, which implies that the peers see each other.

The configuration in lines 3, 4, and 5 suggests that TrdrMary-1 intends to announce the following prefixes to ISP-A-1:

192.200.200.0/26
30.0.0.0/8
160.160.0.0/16
192.168.3.0/24

and the configuration in line 10 suggests that ISP-A-1 intends to announce the following prefix to TrdrMary-1:

0.0.0.0/0

Let’s check the routing tables for these prefixes:

TrdrMary-1#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is 192.100.100.254 to network 0.0.0.0

     192.200.200.0/26 is subnetted, 1 subnets
C       192.200.200.0 is directly connected, Ethernet0
     172.16.0.0/24 is subnetted, 1 subnets
C       172.16.1.0 is directly connected, Ethernet1
     192.168.1.0/32 is subnetted, 1 subnets
C       192.168.1.10 is directly connected, Loopback0
S    192.168.3.0/24 is directly connected, Null0
     192.100.100.0/30 is subnetted, 1 subnets
C       192.100.100.252 is directly connected, Serial1
     160.160.0.0/24 is subnetted, 1 subnets
S       160.160.1.0 is directly connected, Ethernet1
B*   0.0.0.0/0 [20/0] via 192.100.100.254, 00:00:46

TrdrMary-1 does receive the default route (as expected) and modifies its gateway of last resort to 192.100.100.254 (the IP address of ISP-A-1).

ISP-A-1 receives three routes from TrdrMary-1, as shown in lines 16, 17, and 18:

    ISP-A-1#sh ip route
    ...
    Gateway of last resort is 0.0.0.0 to network 0.0.0.0

         192.200.200.0/26 is subnetted, 1 subnets
16  B       192.200.200.0 [20/0] via 192.100.100.253, 00:00:23
17  B    160.160.0.0/16 [20/0] via 192.100.100.253, 00:00:23      
    S*   0.0.0.0/0 is directly connected, Null0
18  B    192.168.3.0/24 [20/0] via 192.100.100.253, 00:20:51      
    ...

However, if you look carefully at ISP-A-1’s routing table, you’ll see that 30.0.0.0/8 (which TrdrMary-1 attempted to announce on line 4) is missing. Why is 30.0.0.0/8 not in ISP-A-1’s routing table? Think on this. We will get back to this question in the next section.