IGP Shortcuts and Routing Table

IGP typically performs two independent computations. The first is performed without considering any LSP. The result of the computation is stored in the inet.0 table. This step is no different from traditional SPF computations and is always performed even if IGP shortcut is disabled.

The second computation is performed considering only LSPs as a logical interface. Each LSP's egress router is considered. The list of destinations whose shortest path traverses the egress router (established during the first computation) is placed in the inet.3 routing table. These destinations are given the egress router of the LSP as a next hop, enabling BGP on the local router to use these LSPs to access BGP next hops beyond the egress router. Normally, BGP can only use LSPs that terminate at the BGP next hop. Note that BGP is the only protocol that uses the inet.3 routing table. Other protocols will not route traffic through these LSPs.

If traffic engineering for IGP and BGP is enabled (see IGP and BGP Destinations), IGP moves all routes in inet.3 into inet.0, merging all routes while emptying the inet.3 table. The number of routes in inet.0 will be exactly the same as before. Route next-hops can traverse a physical interface, an LSP, or the combination of the two if the metrics are equal.