Forwarding Fundamentals

Table 2-1 provided an introductory glance at MPLS through terminology and definitions. This section goes into more depth about how all these concepts come together.

What Is a Label?

Labels, as you can probably guess, are an integral part of Multiprotocol Label Switching. The label allows the decoupling of routing from forwarding, which lets you do all sorts of neat things.

But what is a label? Before we define what a label is, you should know that MPLS can operate in one of two modes:

  • Frame mode

  • Cell mode

Frame Mode

Frame mode is the term used when you forward a packet with a label prepended to the packet in front of the Layer 3 header (the IP header, for example).

RFC 3031, “Multiprotocol Label Switching Architecture,” ...

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