1).
MPLS supports both connection-orientated and connectionless modes of operation,
depending on which signalling protocol is used to establish LSPs. LSPs established using
RSVP-TE, which creates a point-to- point path across the network, or using static
provisioning behave in a connection-orientated fashion. LSPs established using LDP,
which creates a multipoint-to-point path congruent with the shortest path calculated
by the Interior Gateway Protocol, or IGP (the IP routing protocol used within the service
provider??™s network) behave in a connectionless fashion??”although forwarding along the
LSP is still based on label swapping.
RFC 3031 defines the concept of the Forwarding Equivalence Class (FEC). The FEC
(generally pronounced as ???feck??? rather than as ???Eff Eee Cee???) is the set of all packets
that will follow the same path (or set of equivalent paths) through the network.
Packets are classified into FECs at ingress into the network, and the FECs are encoded
into labels??”enabling packets to be forwarded along LSPs. FECs may be arbitrarily
complex??”for example, packets from different source interfaces, with different QoS requirements,
or destined for different IP addresses may be mapped to the same FEC or to
different FECs.
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