IGP Shortcuts

Link-state protocols, such as OSPF and IS-IS, use the SPF algorithm to compute the shortest-path tree to all nodes in the network. The results of such computations can be represented by the destination node, next-hop address, and output interface, where the output interface is a physical interface. LSPs can be used to augment the SPF algorithm. On the node performing the calculations, LSPs appear to be logical interfaces directly connected to remote nodes in the network. If you configure the IGP to treat LSPs the same as a physical interface and to use the LSPs as a potential output interface, the SPF computation results are represented by the destination node and output LSP, effectively using the LSP as a shortcut through the network to the destination.

As an illustration, begin with a typical SPF tree (Figure 4):

Figure 4: Typical SPF Tree, Sourced from Router A

If an LSP connects Router A to Router D and if IGP shortcuts are enabled on Router A, you might have the SPF tree shown in Figure 5.

Figure 5: Modified SPF Tree, Using LSP A-D as a Shortcut

Note that Router D is now reachable through LSP A-D. When computing the shortest path to reach Router D, Router A has two choices:

• Use IGP path A-B-D.
• Use LSP A-D.

Router A decides between the two choices by comparing the IGP metrics for path A-B-D with the LSP metrics for LSP A-D. If the IGP metric is lower, path A-B-D is chosen (Figure 4). If the LSP metric is lower, LSP A-D is used (Figure 5). If both metrics are equal, Router A might share the load between the two paths.

Note that Routers E and F are also reachable through LSP A-D, because they are downstream from Router D in the SPF tree.

Assuming another LSP connects Router A to Router E, you might have the SPF tree shown in Figure 6.

Figure 6: Modified SPF Tree, Using Both LSP A-D and LSP A-E as Shortcuts