| This figure represents the IP packet's
header. L2 is the link-layer header. L3 the IP network
header. Let's see now how an MPLS packet looks like: |
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| As you see this packet has an intermediate
layer header between layer 2 and layer 3 headers. This layer
is called the MPLS layer. |
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| When an IP packet is presented to a
LER router, it pushes the MPLS layer between layer 2 and
layer 3. This way an ordinary IP packet is converted in an
MPLS packet. Let's see now with greater detail how the MPLS layer
header looks like: |
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| The MPLS header is a 32-bit length
header conformed by four parts: 20 bits are used for the label;
3 bits for experimental functions; 1 bit for stack
functions; and 8 bits for the time-to-live field (TTL).
The MPLS header acts as a glue between layer 2 and layer 3 headers. |
