Configuring LSP Metrics

The LSP metric is used to indicate the ease or difficulty of sending traffic over a particular LSP. Lower LSP metric values (lower cost) increase the likelihood of an LSP being used. Conversely, high LSP metric values (higher cost) decrease the likelihood of an LSP being used.

The LSP metric can be specified dynamically by the router or explicitly by the user as described in the following sections:

• Configuring Dynamic LSP Metrics
• Configuring Static LSP Metrics

Configuring Dynamic LSP Metrics

If no specific metric is configured, an LSP attempts to track the IGP metric toward the same destination (the to address of the LSP). IGP includes OSPF, IS-IS, Routing Information Protocol (RIP), and static routes. BGP and other RSVP or LDP routes are excluded.

For example, if the OSPF metric toward a router is 20, all LSPs toward that router automatically inherit metric 20. If the OSPF toward a router later changes to a different value, all LSP metrics change accordingly. If there are no IGP routes toward the router, the LSP raises its metric to 65,535.

Note that in this case, the LSP metric is completely determined by IGP; it bears no relationship to the actual path the LSP is currently traversing. If LSP reroutes (such as through reoptimization), its metric does not change, and thus it remains transparent to users. Dynamic metric is the default behavior; no configuration is required.

Configuring Static LSP Metrics

You can manually assign a fixed metric value to an LSP. Once configured with the metric statement, the LSP metric is fixed and cannot change:

You can include this statement at the following hierarchy levels:

• [edit protocols mpls label-switched-path lsp-name]
• [edit protocols mpls static-label-switched-path lsp-name]
• [edit logical-systems logical-system-name protocols mpls label-switched-path lsp-name]
• [edit logical-systems logical-system-name protocols mpls static-label-switched-path lsp-name]

The LSP metric has several uses:

• When there are parallel LSPs with the same egress router, the metrics are compared to determine which LSP has the lowest metric value (the lowest cost) and therefore the preferred path to the destination. If the metrics are the same, the traffic is shared.

  Adjusting the metric values can force traffic to prefer some LSPs over others, regardless of the underlying IGP metric.

• When an IGP shortcut is enabled (see IGP Shortcuts), an IGP route might be installed in the routing table with an LSP as the next hop, if the LSP is on the shortest path to the destination. In this case, the LSP metric is added to the other IGP metrics to determine the total path metric. For example, if an LSP whose ingress router is X and egress router is Y is on the shortest path to destination Z, the LSP metric is added to the metric for the IGP route from Y to Z to determine the total cost of the path. If several LSPs are potential next hops, the total metrics of the paths are compared to determine which path is preferred (that is, has the lowest total metric). Or, IGP paths and LSPs leading to the same destination could be compared by means of the metric value to determine which path is preferred.

  By adjusting the LSP metric, you can force traffic to prefer LSPs, prefer the IGP path, or share the load among them.

• If router X and Y are BGP peers and if there is an LSP between them, the LSP metric represents the total cost to reach Y from X. If for any reason the LSP reroutes, the underlying path cost might change significantly, but X’s cost to reach Y remains the same (the LSP metric), which allows X to report through a BGP multiple exit discriminator (MED) a stable metric to downstream neighbors. As long as Y remains reachable through the LSP, no changes are visible to downstream BGP neighbors.

It is possible to configure IS-IS to ignore the configured LSP metric by including the ignore-lsp-metrics statement at the [edit protocols isis traffic-engineering shortcuts] hierarchy level. This statement removes the mutual dependency between IS-IS and MPLS for path computation. For more information, see the Junos Routing Protocols Configuration Guide.