Troubleshooting and Verification 67

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ip netmask-format decimal

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MPLS, as you have seen so far, is quite straightforward to configure. MPLS and tag switching need to be enabled both globally and on a per-interface basis. An IGP needs to be running in the core of the network, and BGP needs to be configured between the PE routers.

However, there are many minor problems that can crop up even though MPLS and tag switching are straightforward to configure. The next few sections explain how to deal with and prevent these problems.

IGP Verification

First of all, tag switching or MPLS will not work if an IGP is not configured properly in the service provider core network. In the troubleshooting example, Routing Information Protocol (RIP) version 2 was used as a core IGP. To ensure that RIP is running, use the show ip route command with the rip option to view only RIP routes on the Atlanta POP, Core, and Raleigh POP routers.

The output of the show ip route rip command follows, as executed on the Atlanta POP router:

Atlanta#show ip route rip

204.134.83.0 255.255.255.0 is variably subnetted, 5 subnets, 2 masks

R204.134.83.8 255.255.255.252

[120/1] via 204.134.83.6, 00:00:28, Serial0/0

R204.134.83.3 255.255.255.255

[120/2] via 204.134.83.6, 00:00:28, Serial0/0

R204.134.83.2 255.255.255.255

[120/1] via 204.134.83.6, 00:00:28, Serial0/0

The output of the show ip route rip command follows, as executed on the Core router:

Core#show ip route rip

204.134.83.0 255.255.255.0 is variably subnetted, 5 subnets, 2 masks

68 Chapter 2 Frame-Mode MPLS

R204.134.83.1 255.255.255.255

[120/1] via 204.134.83.5, 00:00:21, Serial0/1

R204.134.83.3 255.255.255.255

[120/1] via 204.134.83.10, 00:00:03, Serial0/0

The output of the show ip route rip command follows, as executed on the Raleigh POP router:

Raleigh#show ip route rip

204.134.83.0 255.255.255.0 is variably subnetted, 5 subnets, 2 masks

R204.134.83.1 255.255.255.255

[120/2] via 204.134.83.9, 00:00:09, Serial0/3

R204.134.83.2 255.255.255.255

[120/1] via 204.134.83.9, 00:00:09, Serial0/3

R204.134.83.4 255.255.255.252

[120/1] via 204.134.83.9, 00:00:09, Serial0/3

An additional step you might want to take in general troubleshooting or verification is to ping each and every network device in the service provider network. By executing the ping command, you are only verifying standard connectivity. If anything is misconfigured here, MPLS or tag switching will not function correctly.

CEF Verification

To ensure that CEF is running, there are two options. Personally, I usually use the show running-configuration command and look for ip cef. An alternative way to verify that CEF is running is to use the show ip cef command. If CEF is not enabled on a device that it should be enabled on, you’ll need to go back to global configuration and execute the ip cef command.

In the network in Figure 2.7, CEF is not enabled on Peer 1 and Peer 2, but it is enabled on the Atlanta POP, Core, and Raleigh POP routers. To verify this, the command show ip cef will be executed on each network device.

The following output shows that CEF is not enabled on Peer 1. (In this output, the Next Hop column contains the router ID of a neighboring device from which the prefix was received.)

Troubleshooting and Verification 69

On the Atlanta POP router, you can verify that CEF is up and running, as shown in the following output:

On the Core router, you can verify that CEF is up and running, as shown in the following output:

70 Chapter 2 Frame-Mode MPLS

204.134.83.9/32 receive 204.134.83.11/32 receive 224.0.0.0/4 drop 224.0.0.0/24 receive 255.255.255.255/32 receive

On the Raleigh POP router, you can verify that CEF is up and running, as shown in the following output:

On Peer 2, you can verify that CEF is not configured, as shown in the following output:

MPLS Verification

To verify that MPLS is enabled, you can either execute a show runningconfiguration command or use the show mpls interfaces command. If