Using IGP shortcuts
This feature allows you to configure a signaled LSP to serve as a shortcut between nodes in an AS. In a shortcut LSP, OSPF includes the LSP in the SPF calculation. When OSPF determines that the LSP shortcut is the best path to a destination, it installs a route into the IP routing table, specifying the LSP tunnel interface as the outbound interface, as well as the cost of the LSP. Only LSPs configured to router IDs can be considered as shortcuts. When the LSP goes down, the LSP tunnel route is removed from the main routing table.
The cost of the LSP is the user-configured metric for the LSP. When there is no user-configured metric, the underlying IP cost of the LSP is used. For example, when the IP cost of the best underlying path between two routers is 2, and there is an LSP configured between these two routers, the cost of the LSP is 2. Once an LSP is used as a next hop for a destination, the cost of the LSP can be used to calculate other destinations that can use the LSP egress node as next hop. This allows traffic for addresses downstream from the LSP egress node (including prefixes of the egress node) to use the LSP shortcut.
When OSPF is already using an LSP tunnel route to an Area Border Router (ABR), all inter-area routes through that ABR use the LSP as the next hop, provided there are no other better paths to the destination (paths through other ABRs). An LSP to a destination outside an area is not used by OSPF in the calculation of inter-area routes.