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Configuring IS-IS to Use IGP Shortcuts

IS-IS always performs SPF calculations to determine next hops. For prefixes reachable through a particular next hop, IS-IS places that next hop for that prefix in the inet.0 routing table. In addition, for routers running MPLS, IS-IS installs the prefix for IPv4 routes in the inet.3 routing table as well. The inet.3 table, which is present on the ingress router, contains the host address of each MPLS label-switched path (LSP) egress router. BGP uses this routing table to resolve next-hop addresses.

Note: Whenever possible, use IS-IS IGP shortcuts instead of traffic engineering shortcuts.

If you enable IS-IS traffic engineering shortcuts and if there is a label-switched path to a point along the path to that prefix, IS-IS installs the prefix in the inet.3 routing table and uses the label-switched path as a next hop. The net result is that for BGP egress routers for which there is no label-switched path (LSP), BGP automatically uses an LSP along the path to reach the egress router.

Beginning with JUNOS Release 9.3, IS-IS traffic engineering shortcuts support IPv6 routes. LSPs to be used for shortcuts continue to be signaled using IPv4. However, by default, shortcut routes calculated through IPv6 routes are added to the inet6.3 routing table. The default behavior is for only BGP to use LSPs in its calculations. If you configure MPLS so that both BGP and interior gateway protocols use LSPs for forwarding traffic, shortcut routes calculated through IPv6 are added to the inet6.0 routing table.

RSVP LSPs with a higher preference than IS-IS routes are not considered during the computation of traffic engineering shortcuts.

To configure IS-IS so that it uses label-switched paths as shortcuts when installing information in the inet.3 or inet6.3 routing table, include the following statements:




traffic-engineering {



family inet {



shortcuts;
multicast-rpf-routes;
}


}





family inet6 {
shortcuts;
}


}

For IPv4 traffic, include the inet statement. For IPv6 traffic, include the inet6 statement.

To ignore the metric of RSVP LSPs in shortcut decisions, include the ignore-lsp-metrics statement:



traffic-engineering {

ignore-lsp-metrics;
}

This option avoids mutual dependency between IS-IS and RSVP, eliminating the time period when the RSVP metric used for shortcuts is not up to date.

For a list of hierarchy levels at which you can include these statements, see the statement summary sections for these statements.

Because the inet.3 routing table is present only on ingress routers, you can configure label-switched path shortcuts only on these routers.

For more information about configuring label-switched paths and MPLS, see the JUNOS MPLS Applications Configuration Guide.

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