Checking the BPG Layer

Purpose

After you have configured the label-switched path (LSP) and determined that it is up, and configured BGP and determined that sessions are established, ensure that BGP is using the LSP to forward traffic.

Figure 23 illustrates the BGP layer of the layered MPLS model.

Figure 23: Checking the BGP Layer

When you check the BGP layer, you verify that the route is present and active, and more importantly, you ensure that the next hop is the LSP. There is no point in checking the BGP layer unless the LSP is established, because BGP uses the MPLS LSP to forward traffic. If the network is not functioning at the BGP layer, the LSP does not work as configured.

Figure 24 illustrates the MPLS network used in this chapter.

Figure 24: MPLS Network Broken at the BGP Layer

The network shown in Figure 24 is a fully meshed configuration where every directly connected interface can receive and send packets to every other similar interface. The LSP in this network is configured to run from ingress router R1, through transit router R3, to egress router R6. In addition, a reverse LSP is configured to run from R6 through R3 to R1, creating bidirectional traffic.

The cross shown in Figure 24 indicates where BGP is not being used to forward traffic through the LSP. Possible reasons for the LSP not working correctly are that the destination IP address of the LSP does not equal the BGP next hop or that BGP is not configured properly.

Steps To Take

To check the BGP layer, follow these steps:

1. Check That BGP Traffic Is Using the LSP
2. Check BGP Sessions
3. Verify the BGP Configuration
4. Examine BGP Routes
5. Verify Received BGP Routes
6. Take Appropriate Action
7. Check That BGP Traffic Is Using the LSP Again