SR OS supports carrying over of the operational bandwidth (for example, the last successfully signaled bandwidth) of an LSP path to the next active path following a switchover. The new active path can be a secondary or a primary path. The bandwidth is not lost even when the previously active path fails. The last successfully signaled bandwidth is known as the last adjusted bandwidth.
This feature is enabled using the configure router mpls lsp auto-bandwidth use-last-adj-bw command.
When enabled, secondary paths are initially signaled with the last adjusted bandwidth of the primary, and not the configured bandwidth. If signaling a secondary at this bandwidth fails after some number of retries, then the path fails instead of falling back to using the configured bandwidth. The number of retries of secondary paths at the last adjusted bandwidth is configured using the secondary-retry-limit command under use-last-adj-bw.
A shutdown of the primary or any configuration change events that cause a switch to a secondary, uses the last adjusted bandwidth. The user can toggle use-last-adj-bw at any time; this does not require an administrative shutdown of auto bandwidth, however, the new value is not used until the next path switchover.
The last adjusted bandwidth value is reset on a shutdown of MPLS, the LSP, or autobandwidth.
If the revert timer is enabled, the primary is resignaled before the revert timer expires with its configured bandwidth. An auto-bandwidth MBB using the last adjusted bandwidth of the secondary occurs immediately on switching back when the revert timer expires. If the system switches to a new path while an auto-bandwidth MBB is in progress on the previously active path, then the bandwidth used to signal the new path is the new value that was being attempted on the old path (instead of the last adjusted bandwidth). This means that the new path establishes with the most up to date bandwidth for the LSP (provided sufficient network resources are available) instead of a potentially out of date bandwidth.