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Packet Forwarding Based
on Label Swapping
The packet forwarding process at each router is
based on the concept of label swapping. This concept is similar to
what occurs at each Asynchronous Transfer Mode (ATM) switch in a permanent
virtual circuit (PVC). Each MPLS packet carries a 4-byte encapsulation
header that contains a 20-bit, fixed-length label field. When a packet
containing a label arrives at a router, the router examines the label
and copies it as an index to its MPLS forwarding table. Each entry
in the forwarding table contains an interface-inbound label pair mapped
to a set of forwarding information that is applied to all packets
arriving on the specific interface with the same inbound label.
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