BGP

Chapter 5. Getting Started with BGP

In this chapter I’ll take a basic non-BGP Cisco router configuration (see Appendix A for an example configuration) and turn it into a full BGP configuration for a small, multihomed network, step by step. Figure 5-1 shows the Autonomous Systems and the network connections between them that are used for examples in this chapter.

Figure 5-1. Example network multihomed to two ISPs

Enabling BGP

It’s common to use ip unnumbered for point-to-point (leased line) connections to an ISP, as shown in Example 5-1.

Example 5-1. Unnumbered configuration for a leased line

!
interface Ethernet0
 ip address 192.0.2.65 255.255.255.192
!
interface Serial0
 description ISP A
 ip unnumbered Ethernet0
!
ip route 0.0.0.0 0.0.0.0 Serial0
!

Without BGP, there is no need to assign a small subnet to the leased line “network,” because the remote system is already uniquely identified by the fact that it’s the only system at the other side of the point-to-point link. There must still be an IP address present to enable IP processing for the interface, however. The ip unnumbered command “borrows” the address from another interface (in this case Ethernet0) for this purpose. All traffic to the entire Internet is then directed to the Serial0 interface by using the command ip route 0.0.0.0 0.0.0.0 Serial0 to create a default route, without any need to know the actual address of the router on ...