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Chapter 7. MPLS Troubleshooting

This chapter includes the following topics:

Previous chapters within this book discuss the theory of Multiprotocol Label Switching (MPLS) and its configuration on Cisco IOS Software and include a migration and configuration case study. This chapter focuses on the monitoring and troubleshooting aspects of MPLS.

MPLS is simple to configure and troubleshoot—few things can go wrong. Recent IOS versions (for example, IOS release 12.2) check most of the prerequisites for successful MPLS operation before they enable you to enter MPLS-related configuration commands.


MPLS is so transparent in simple IP networks that do not use advanced MPLS features (for example, Border Gateway Protocol [BGP] running only on the edge routers or MPLS applications, such as MPLS Virtual Private Networks [VPN] or MPLS Traffic Engineering) that it's sometimes hard to detect that MPLS is not operational. The first problems usually arise when you try to deploy advanced MPLS features.

However, should you encounter MPLS-related problems in your network, the following list outlines the things that you should check first. (The following section contains detailed instructions for performing these checks.)

  • Is Cisco Express Forwarding (CEF) enabled? MPLS does not work without CEF because CEF is the only switching mechanism that can provide the necessary forwarding structures required by the MPLS label imposition component. CEF needs to be enabled globally on all interfaces receiving unlabeled IP datagrams.

  • Is MPLS enabled on all routers?

  • Is MPLS disabled on any of the interfaces?

If the first quick checks fail to provide you with an answer, you need to go into more in-depth MPLS troubleshooting. In line with the MPLS architecture, you need to perform the following two levels of troubleshooting:

  • Control plane— Is a TDP or an LDP session established between adjacent devices and are labels exchanged between them?

  • Data plane— Are labeled packets propagated across the MPLS network?


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