Multiprotocol Label Switching (MPLS) integrates the performance and traffic management capabilities of data link layer (Layer 2) switching with the scalability, flexibility and performance of network layer (Layer 3) routing. MPLS has a data plane that forwards packets based on fixed length labels. MPLS also has a control plane. This deals with label assignment and distribution between the devices based on the Forward Equivalence Class (FEC). This could be based on the destination IP address, traffic belonging to a particular Virtual Private Network (VPN) or traffic requiring a specific Quality of Service (QoS). Routers at the edge of an MPLS cloud receive normal IP packets, attach a label or stack of labels and forward it into the MPLS cloud.
These common problems occur with basic MPLS:
The Tag Distribution Protocol (TDP) or Label Distribution Protocol (LDP) session does not start.
The TDP or LDP session starts, but the labels are not allocated or distributed.
The labels are allocated and distributed, but the forwarded packets are not labeled.
The large IP packets are not propagated across the MPLS backbone, even though they were successfully propagated across the pure IP backbone.
Cisco IOS Software has many commands for resolving MPLS issues. However, you must first ensure that the routing protocol being used behaves correctly and that the routes are being exchanged properly.
To solve MPLS neighbor related problems, perform these options:
Check whether TDP or LDP is enabled on the routers and that the neighbors use the same protocol. TDP and LDP are not compatible with each other.
Diagnose problems related to label distribution and the forwarding of labeled packets by ensuring that Cisco Express Forwarding (CEF) switching is enabled on the routers. CEF is a prerequisite for MPLS to work.
Make sure that MPLS is enabled globally as well as under the specific interfaces on the device.
Make sure that the label bindings are exchanged and used for the various routes by verifying the Forwarding Information Base (FIB), Label Information Base (LIB) and Label Forwarding Information Base (LFIB) tables.
In some cases, basic MPLS may work. However, in other MPLS applications that use multiple labels, MPLS may fail. This could be due to Maximum Transmission Unit (MTU) issues, where switches with no support for jumbo frames are available in the forwarding path. To make sure that the labels are being used to reach the destination and that the Label Switched Path (LSP) is working, use the MPLS-aware traceroute functionality.
Other issues related to specific MPLS applications, like VPNs and Traffic Engineering (TE), may require more extensive problem solving related to the specific mechanism.