IGP Shortcut Applications
You can use shortcuts to engineer traffic traveling toward destination nodes that do not support MPLS LSPs. For example, in Modified SPF Tree, Using LSP A–D and LSP A–E as Shortcuts, traffic traveling toward Router F enters LSP A–E. You can control traffic between Router A and Router F by manipulating LSP A–E; you do not need to explicitly set up an LSP between Router A and Router F.
In Figure 1, all traffic from Region 1 to Region 2 traverses LSP A–B if IGP shortcuts are enabled on the ingress router (Router A), permitting aggregation of interregional traffic into one LSP. To perform traffic engineering on the interregional traffic, you have to manipulate LSP A-B only, which avoids creating n2 LSPs from all routers in Region 1 to all routers in Region 2 and allows efficient resource controls on the backbone network.
Shortcuts allow you to deploy LSPs into a network in an incremental, hierarchical fashion. In Figure 2, each region can choose to implement traffic engineering LSPs independently, without requiring cooperation from other regions. Each region can choose to deploy intraregion LSPs to fit the region’s bandwidth needs, at the pace appropriate for the region.
When intraregion LSPs are in place, interregional traffic automatically traverses the intraregion LSPs as needed, eliminating the need for a full mesh of LSPs between edge routers. For example, traffic from Router A to Router D traverses LSPs A–B, B–C, and C–D.