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Configuring Primary and Secondary LSPs

By default, an LSP routes itself hop-by-hop toward the egress router. The LSP tends to follow the shortest path as dictated by the local routing table, usually taking the same path as destination-based, best-effort traffic. These paths are “soft” in nature because they automatically reroute themselves whenever a change occurs in a routing table or in the status of a node or link.

To configure the path so that it follows a particular route, create a named path using the path statement, as described in Configuring the Ingress Router for MPLS-Signaled LSPs. Then apply the named path by including the primary or secondary statement. A named path can be referenced by any number of LSPs.

To configure primary and secondary paths for an LSP, complete the steps in the following sections:

Configuring Primary and Secondary Paths for an LSP

The primary statement creates the primary path, which is the LSP’s preferred path. The secondary statement creates an alternative path. If the primary path can no longer reach the egress router, the alternative path is used.

To configure primary and secondary paths, include the primary and secondary statements:

You can include these statements at the following hierarchy levels:

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

When the software switches from the primary to a secondary path, it continuously attempts to revert to the primary path, switching back to it when it is again reachable, but no sooner than the retry time specified in the retry-timer statement. (For more information, see Configuring the Connection Between Ingress and Egress Routers.)

You can configure zero or one primary path. If you do not configure a primary path, the first secondary path that is established is selected as the path.

You can configure zero or more secondary paths. All secondary paths are equal, and the software tries them in the order that they are listed in the configuration. The software does not attempt to switch among secondary paths. If the current secondary path is not available, the next one is tried. To create a set of equal paths, specify secondary paths without specifying a primary path.

If you do not specify any named paths, or if the path that you specify is empty, the software makes all routing decisions necessary to reach the egress router.

Configuring the Revert Timer for LSPs

For LSPs configured with both primary and secondary paths, it is possible to configure the revert timer. If a primary path goes down and traffic is switched to the secondary path, the revert timer specifies the amount of time (in seconds) that the LSP must wait before it can revert traffic back to a primary path. If during this time, the primary path experiences any connectivity problems or stability problems, the timer is restarted. You can configure the revert timer for both static and dynamic LSPs.

The Junos OS also makes a determination as to which path is the preferred path. The preferred path is the path that has not encountered any difficulty in the last revert timer period. If both the primary and secondary paths have encountered difficulty, neither path is considered preferred. However, if one of the paths is dynamic and the other static, the dynamic path is selected as the preferred path.

If you have configured BFD on the LSP, Junos OS waits until the BFD session comes up on the primary path before starting the revert timer counter.

The range of values you can configure for the revert timer is 0 through 65,535 seconds. The default value is 60 seconds.

If you configure a value of 0 seconds, the traffic on the LSP, once switched from the primary path to the secondary path, remains on the secondary path permanently (until the network operator intervenes or until the secondary path goes down).

You can configure the revert timer for all LSPs on the router at the [edit protocols mpls] hierarchy level or for a specific LSP at the [edit protocols mpls label-switched-path lsp-name] hierarchy level.

To configure the revert timer, include the revert-timer statement:

For a list of hierarchy levels at which you can include this statement, see the summary section for this statement.

Specifying the Conditions for Path Selection

When you have configured both primary and secondary paths for an LSP, you may need to ensure that only a specific path is used.

The select statement is optional. If you do not include it, MPLS uses an automatic path selection algorithm.

• manual—The path is immediately selected for carrying traffic as long as it is up and stable. Traffic is sent to other working paths if the current path is down or degraded (receiving errors). This parameter overrides all other path attributes except the select unconditional statement.
• unconditional—The path is selected for carrying traffic unconditionally, regardless of whether the path is currently down or degraded (receiving errors). This parameter overrides all other path attributes.

  Because the unconditional option switches to a path without regard to its current status, be aware of the following potential consequences of specifying it:

  • If a path is not currently up when you enable the unconditional option, traffic can be disrupted. Ensure that the path is functional before specifying the unconditional option.
  • Once a path is selected because it has the unconditional option enabled, all other paths for the LSP are gradually cleared, including the primary and standby paths. No path can act as a standby to an unconditional path, so signaling those paths serves no purpose.

For a specific path, the manual and unconditional options are mutually exclusive. You can include the select statement with the manual option in the configuration of only one of an LSP’s paths, and the select statement with the unconditional option in the configuration of only one other of its paths.

Enabling or disabling the manual and unconditional options for the select statement while LSPs and their paths are up does not disrupt traffic.

• [edit protocols mpls label-switched-path lsp-name (primary | secondary) path-name]
• [edit logical-systems logical-system-name protocols mpls label-switched-path lsp-name (primary | secondary) path-name]