Troubleshooting guide

BGP Overview

For example, in Figure 1 on page 7 the provider’s edge routers connect to two independent customers,

Customer A and Customer B. Each customer would like to communic

ate private information between their

own respective sites. Customer A Site 1 uses an IPv4 network address of 192.168.1.0 /24 and Customer A

Site 2 uses an IPv4 network address of 192.168.2.0 /24. These identical network addresses are also used for

Customer B Sites 1 and 2, respectively. The provider’s router must be able to associate one 192.168.1.0 /24

with the public address for Customer A Site 1 and the other with Customer B Site 1. Likewise, separate

associations for 192.168.2.0 /24 for Customer A and B Site 2 must be maintained.

The provider’s edge router separates routes by the physical or logical

interface on which they arrive. The

router then stores routes from each customer in a separate VRF routing table. Different customers’ routing

tables cannot be combined.

The service provider edge router connecting to the local

site forms an MPLS label switched path (LSP)

with the service provider edge router connecting to the authorized remote site. (An LSP resembles a

dynamic permanent virtual circuit (PVC).) The edge routers mark packets with an MPLS label that directs

them toward the other router through the LSP so that only Customer A sites receive Customer A routes.

MPLS

192.168.1.0 /24

192.168.2.0 /24

192.168.1.0 /24

192.168.2.0 /24

VRF Customer A VRF Customer B

Customer A Site 1

Customer A Site 2

Customer B Site 1

Customer B Site 2

Edge Router

Router Router

Router

Figure 1. Traffic Separation Using VRF

Multihoming

Multihoming is when a router has more than one connection to the Internet. BGP is especially useful for

this type of application. When a router has only one Internet connection, static routing is normally

implemented. The lower overhead and simpler configuration outweigh the loss in control. It does not

matter that the router cannot balance loads because all external traffic has the same destination.