This section describes how MPLS (Multiprotocol Label Switching) and IP routing work together to forward information on your network.
MPLS provides a great deal of flexibility for routing packets. Received IP unicast frames can be routed normally or tunneled through LSPs. If a matching FEC exists for a received packet, the packet may be transmitted using an LSP that is associated with the FEC. The packet is encapsulated using an MPLS shim header before being transmitted.
Received MPLS packets can be label switched or routed normally toward the destination. Packets that are in the middle of an LSP are label switched. The incoming label is swapped for a new outgoing label and the packet is transmitted to the next LSR. For packets that have arrived at the end of an LSP (the egress end of the LSP), the label is popped. If this label is the bottom of the stack, the shim header is stripped and the packets are routed to the destination as normal IP packets.
An MPLS domain is generally defined to be an OSPF (Open Shortest Path First) or IS-IS autonomous system (AS). You can use MPLS to reach destinations inside one of these AS types. You can also use MPLS to tunnel through all or part of an AS in order to reach destinations outside of the AS.
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