• Supported Platforms
• M Series
• T Series

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, for the purposes of resolving BGP next hops. 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 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 (see Figure 1).

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 2.

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 1). If the LSP metric is lower, LSP A–D is used (Figure 2). If both metrics are equal, LSP A–D is chosen because LSP paths are preferred over IGP 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 that another LSP connects Router A to Router E, you might have the SPF tree shown in Figure 3.

This section discusses the following topics:

• Enabling IGP Shortcuts
• LSPs Qualified in Shortcut Computations
• IGP Shortcut Applications
• IGP Shortcuts and Routing Table
• IGP Shortcuts and VPN Environments