Foundation Topics

The configuration of MPLS on a per-router basis is relatively straightforward. That is, the basic configuration is straightforward. There are a number of considerations that must be taken into account. These include the size of the network and the number of prefixes to be propagated.

MPLS and its associated overhead may be a significant hit to the resources of a router. In a typical service provider model, the router should expect to hold the full Internet routing table, which is well in excess of 200,000 prefixes. Typically the service provider runs an Interior Gateway Protocol (IGP) such as Intermediate System-to-Intermediate System (IS-IS) and an Exterior Gateway Protocol (EGP) such as Border Gateway Protocol (BGP). Each protocol will maintain a number of prefixes. BGP will contain all of the publicly advertised prefixes while the IGP will contain the internal destinations of the service provider network. This is a daunting amount of information to keep current. Add to that the need for Cisco Express Forwarding (CEF) information and the MPLS required Forwarding Information Base (FIB), Label Information Base (LIB), and Label Forwarding Information Base (LFIB), along with adjacency information required for IGP, EGP, and MPLS, and it is easy to see where the router may become rather overloaded.

MPLS is typically enabled on an interface-by-interface basis. There are global commands as well, certainly. The idea behind frame mode MPLS is to impose labels between the Layer 2 and Layer 3 headers. This can cause problems in large packets. The addition of four additional bytes of information might cause the frame size to exceed the Maximum Transmission Unit (MTU) defined for the interface. This will preclude such frames from traversing the router.

The procedure for enabling MPLS consists of only a few steps:

Step 1 Configure CEF—CEF must be enabled as a prerequisite to enabling MPLS.

Step 2 Configure MPLS on a Frame Mode Interface—MPLS backbone interfaces should be MPLS enabled.

Step 3 Configure MTU sizes as needed—To prevent frames from exceeding MTU size on the interface, the MTU should be manually adjusted in MPLS-enabled interfaces.

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