The second option has been used because there are too many existing IP routing protocols that would have to be modified to carry labels

«2003, Chco Systems, Inc. All right* reserved. MPLS v20-2-4

One application of MPLS is unicast IP routing. A label is assigned to destination IP networks and is later used to label packets sent toward those destinations.

Note In MPLS terminology, a forwarding equivalence class (FEC) equals an IP destination network.

Standard or vendor-specific routing protocols are used to advertise IP routing information. MPLS adds a new piece of information that must be exchanged between adjacent routers.

There are two possible approaches to propagating this additional information (labels) between adjacent routers:

■ Extend the functionality of existing routing protocols

■ Create a new protocol dedicated to exchanging labels

The first approach requires much more time and effort because of the large number of different routing protocols (Open Shortest Path First, or OSPF; Intermediate System-to-Intermediate System, or IS-IS; Enhanced Interior Gateway Routing Protocol, or EIGRP; Interior Gateway Routing Protocol, or IGRP; Routing Information Protocol, or RIP; and so on). The approach also causes interoperability problems between routers that support this new functionality and those that do not. Therefore, the Internet Engineering Task Force (IETF) selected the second option.

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