OSPF and IS-IS: Choosing an IGP for Large-Scale Networks

11.2. Traffic Engineering: An Overview

Key to the usefulness of MPLS is that after the LSP is established, your EGP or IGP or both can view the LSP as a traffic path when calculating best paths to a destination. This is again similar to an ATM or Frame Relay VC: Even though the path actually traverses multiple switching nodes, a routing protocol can view it as a single link between the ingress node and the egress node.

The example LSP you have seen so far in Figures 11.2 and 11.3 is too simple to be of practical interest. There is only one physical route from the ingress LSR to the egress LSR, so routing protocols are going to choose it whether an LSP exists or not. But consider the ingress and egress nodes in Figure 11.6. This network contains ...