MPLS Forwarding
The MPLS architecture and protocols were developed in the late 1990s in response to
concerns about the limitations of connectionless IP forwarding. Specific issues identified
in the MPLS architecture and documented in RFC 3031 include
?– It is easier to implement label lookup and replacement than the IP forwarding
algorithms. This was one of the initial drivers for MPLS (though its importance
has often been overstated) since label switching was implemented in hardware
(e.g., in ATM switches) at a time when IP forwarding was only implemented in
software??”and hence label switches could support higher packet forwarding rates
than IP routers. Carrier-class IP routers today are able to forward packets at line
rate using hardware switching, so this issue is no longer considered important
(though it is still true that label switching is simpler than IP forwarding).
?– The ingress router may use information not included in the packet header (and
which would thus not be available to a subsequent router) when assigning a packet
to an LSP. Because subsequent routers (Label Switching Routers or LSRs) along the
LSP need to examine only the label, and not the IP header, this can be generalized to
enable non-IP traffic to be forwarded on MPLS LSPs (as with Ethernet over MPLS).
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