Specifying Path Selection

When you have configured both primary and secondary paths for an LSP, it might be necessary to ensure that only a specific path is used.

The select statement is optional. If you do not configure this statement, an automatic path selection algorithm is used. If you configure the select statement, you must specify either the manual option or the unconditional option. You cannot specify both.

The manual and unconditional options do the following:

• manual—The path is selected for carrying traffic if it is up and stable for at least the revert timer window (potentially before the revert timer has elapsed). 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 when using this parameter:

• If a path is not currently up when you enable the unconditional option, it can cause traffic disruptions. Ensuring that a path is functional before specifying the unconditional option helps to avoid this issue.
• 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.

You can configure only one path for an LSP with the select manual statement and one path for an LSP with the select unconditional statement. These statements cannot be configured on two or more paths used by the same LSP.

You can enable and disable the manual and unconditional options for the select statement while LSPs and their paths are up and running without disrupting traffic.

To modify the behavior of the LSP such that a path is selected for carrying traffic if it is up and stable for at least the revert timer window, include the select statement with the manual option:

select {

    manual;

}

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

To modify the behavior of the LSP such that a path is selected for carrying traffic unconditionally, regardless of whether the path is currently down or degraded, include the select statement with the unconditional option:

select {

    unconditional;

}

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