Configuring Path Selection

The user can exercise control over the paths used by an LSP by setting the select mode and by specifying a preferred path using the select-path command.

By default, an LSP with primary and secondary paths configured immediately uses the primary path. When the primary path fails, a secondary (redundant) path is used. When the primary path comes back up, traffic reverts to the primary path and the secondary (redundant) path returns to a back-up state. However, path selection can be configured to operate in any of the following three modes:

auto select mode - This is the default mode of the router and no special configuration is required. When this mode is operating, the router always tries to use the primary path to carry traffic when the primary path has stayed operating in the working state for at least the amount of time specified in revert-timer configuration command. When no revert-timer is configured for the LSP, a value of zero seconds is used which causes immediate switching of the path.
manual select mode - In this mode, traffic is switched to a user-specified path after the selected path has stayed operating in the working state for at least the amount of time specified in the revert-timer configuration. In the manual select mode, traffic stays on the selected path as long as the path remains in working condition and only switches to an alternative path, such as the primary path when the selected path experiences a failure. Once the selected path comes back into working condition for the amount of time specified by the revert-timer configuration, traffic is switched back.

When an LSP is configured in manual select path mode with at least one other hot standby secondary path, the operation is as follows: when the selected path goes down, the system tries to bring up one hot standby secondary path to protect the primary path, but when the selected path is up, system brings down the hot standby secondary path since the selected path is already serving as a hot standby for the primary path.

unconditional select mode - In this mode, traffic is switched to and stays on the selected path regardless of the path‘s condition even when it is in a failure state. The main difference between manual and unconditional select mode is the test of the working condition of the user selected path. When configured in unconditional mode, the router starts the signaling for the selected path if has not already done so and brings down all other paths; this includes the primary path and the path carrying traffic when it is not the selected path. Because the speed at which the selected path comes up cannot be guaranteed, traffic forwarding might be disrupted.

The auto select mode and manual select mode configurations use the revert-timer configuration.

The following steps configure the LSP named samplelsp with a primary path named pathprimary and two secondary paths named pathsecondarya and pathsecondaryb . The path named pathsecondaryb is configured as a selected path in the manual select mode.

Enable the device for configuration.
```
device# configure
```
Enable the MPLS router.
```
device(config)# router mpls
```
Enable and configure the signaled label switched path (LSP).
```
device(config-router-mpls)# lsp samplelsp
```
In this example, the LSP is called samplelsp .
Configure the primary path.
```
device(config-router-mpls-lsp-samplelsp)# primary-path pathprimary
```
In this example, the primary path selected is called pathprimary .
Configure the first secondary path.
```
device(config-router-mpls-lsp-samplelsp)# secondary-path pathsecondarya
```
In this example the first primary path configured is called pathsecondarya.
Configure the second secondary path.
```
device(config-router-mpls-lsp-samplelsp)# secondary-path pathsecondaryb
```
In this example, the secondary path configured is called pathsecondaryb.

Configure the selected path in the manual mode.

device(config-router-mpls-lsp-samplelsp)# select-path manual pathsecondaryb

Apply the parameter modifications to the LSP.
```
device(config-router-mpls-lsp-samplelsp)# commit
```
After configuring this example, traffic for samplelsp travels over the pathsecondaryb path whenever this path is in working condition because the no revert-timer has been configured. When a revert-timer is configured, the router waits for the pathsecondaryb path to be up for at least the amount of time specified in the configuration of the revert-timer command. When the select mode is changed to unconditional, as shown below, traffic is switched to the pathsecondaryb path, regardless of its working condition.

Change the select mode to unconditional for path pathsecondaryb.

device(config-router-mpls-lsp-samplelsp)# select-path unconditional pathsecondaryb

Example

The following example configures the LSP named samplelsp with a primary path named pathprimary and two secondary paths named pathsecondarya and pathsecondaryb . The path named pathsecondaryb is configured as a selected path in the manual select mode. After the parameters modification are made to the LSP, the select mode is changed to unconditional, and the traffic is switched to the pathsecondaryb path.

device# configure
device(config)# router mpls
device(config-router-mpls)# lsp samplelsp
device(config-router-mpls-lsp-samplelsp)# primary-path pathprimary
device(config-router-mpls-lsp-samplelsp)# secondary-path pathsecondarya
device(config-router-mpls-lsp-samplelsp)# secondary-path pathsecondaryb
device(config-router-mpls-lsp-samplelsp)# select-path manual pathsecondaryb
device(config-router-mpls-lsp-samplelsp)# commit
device(config-router-mpls-lsp-samplelsp)# select-path unconditional pathsecondaryb

Configuration changes made to the select mode do not take effect for an already enabled LSP until the change is activated implicitly using the commit command, explicitly using a reoptimize command, or a system reboot is performed.

Note

When the user configures a primary path to be the selected path, a message is generated that states that it is already the default system behavior because the primary path is the default preferred path. In this instance, no configuration information is saved in the configuration file.