IGP Shortcut and Forwarding Adjacency

The RSVP-TE LSP or SR-TE LSP shortcut for IGP route resolution supports packet forwarding to IGP learned routes using an RSVP-TE LSP. This is also referred to as IGP shortcut. This feature instructs IGP to include RSVP-TE LSPs and SR-TE LSPs that originate on this node and terminate on the router ID of a remote node as direct links with a metric equal to the metric provided by MPLS. During the IP reach to determine the reachability of nodes and prefixes, LSPs are overlaid and the LSP metric is used to determine the subset of paths that are equal to the lowest cost to reach a node or a prefix. When computing the cost of a prefix that is resolved to the LSP, if the user enables the relative-metric option for this LSP, the IGP applies the shortest IGP cost between the endpoints of the LSP, plus the value of the offset, instead of using the LSP operational metric.

When a prefix is resolved to a tunnel next hop, the packet is sent labeled with the label stack corresponding to the NHLFE of the RSVP LSP and the explicit-null IPv6 label at the bottom of the stack in the case of an IPv6 prefix. Any network event causing an RSVP LSP to go down triggers a full SPF computation which may result in installing a new route over another RSVP LSP shortcut as tunnel next hop or over a regular IP next hop.

When igp-shortcut is enabled at the IGP instance level, all RSVP-TE and SR-TE LSPs originating on this node are eligible by default as long as the destination address of the LSP, as configured in config>router>mpls>lsp>to, corresponds to a router-id of a remote node. LSPs with a destination corresponding to an interface address or any other loopback interface address of a remote node are automatically not considered by IS-IS or OSPF. The user can, however, exclude a specific RSVP-TE LSP or a SR-TE LSP from being used as a shortcut for resolving IGP routes as described in IGP Shortcut Feature Configuration.

It is specifically recommended to disable the igp-shortcut option on RSVP LSP which has the cspf option disabled unless the full explicit path of the LSP is provided in the path definition. MPLS tracks in RTM the destination or the first loose-hop in the path of a non CSPF LSP and therefore this can cause bouncing when used within IGP shortcuts.

The SPF in OSPF or IS-IS only uses RSVP LSPs as forwarding adjacencies, IGP shortcuts, or as endpoints for LDP-over-RSVP. These applications of RSVP LSPs are mutually exclusive at the IGP instance level. If two or more options are enabled in the same IGP instance, forwarding adjacency takes precedence over the shortcut application which takes precedence over the LDP-over-RSVP application. The SPF in IGP uses SR-TE LSPs as IGP shortcuts only.

Table 1 summarizes the outcome in terms of RSVP LSP role of mixing these configuration options.

The igp-shortcut shutdown command disables the resolution of IGP routes using IGP shortcuts.