Advertising LSPs into IGPs
You can configure your IGP to treat an LSP as a link. IGP shortcuts allow only the ingress router of an LSP to use the LSP in its SPF computation. However, other routers on the network do not know of the existence of that LSP, so they cannot use it. This can lead to suboptimal traffic engineering. In addition, only BGP can use an IGP shortcut to an LSP. When you advertise an LSP as a link into the IGP, all traffic can traverse it, and all routers know about it.
As an example, consider the network shown in Figure 1.
Assume that Router A is computing a path to Router D. The link between Router E and Router F has a metric of 20; all other links have a metric of 10. Here, the path chosen by Router A is A–B–C–D, which has a metric of 30, instead of A–E–F–D, which has a metric of 40.
If Router E has an LSP to Router D with a metric of 15, you want traffic from Router A to Router D to use the path A–E–D, which has a metric of 25, instead of the path A–B–C–D. However, because Router A does not know about the LSP between Router E and Router D, it cannot route traffic through this path.
For all routers on the network to know about the LSP between Router E and Router D, you need to advertise it. This advertisement announces the LSP as a unidirectional, point-to-point link in the link-state database, and all routers can compute paths using the LSP. The link-state database maintains information about the AS topology and contains information about the router’s local state (for example, the router’s usable interfaces and reachable neighbors). In Figure 1, Router A will see the link from Router E to Router D and route traffic along this lower-metric path.
Because an LSP is announced as a unidirectional link, you might need to configure a reverse LSP (one that starts at the egress router and ends at the ingress router) so that the SPF bidirectionality check succeeds. As a step in the SPF computation, IS-IS considers a link from Router E to Router D. Before IS-IS uses any link, it verifies that there is a link from Router D to Router E (there is bidirectional connectivity between router E and D). Otherwise, the SPF computation will not use an announced LSP.
When an LSP is advertised to the IGP, the advertising router uses the LSP as the forwarding path for regular routes after installing them in the inet.0 routing table. All packets traversing the router could be forwarded through the LSP. Conversely, IGP shortcuts are used only to forward packets that are following BGP routes.
Do not configure IGP shortcuts and advertise LSPs to the IGP at the same time.