What path control algorithms does NSP support?

Star algorithm

NSP provides a load-balancing and optimal-path-placement algorithm, known as the STAR algorithm. This algorithm uses an internal metric, calculated from the current value of the TE bandwidth reservation, to route the CSPF paths. Every path that is allocated on a TE link changes the internal metric for both the link and the overall path. Initially, all links have the same star weight, or metric, so the first path requests for CSPF traversal will choose the shortest path that satisfies all constraints. If there are multiple paths that satisfy the user constraints, then a path will be chosen randomly. This behavior is the same for normal CSPF.

Subsequent requests will choose paths that possess the least star weight, thereby ignoring the path that the normal CSPF algorithm would have chosen. The calculation of the star weight is based on a formula that uses the current link reservation. The user constraints are still satisfied. This balances the overall network utilization.

The STAR algorithm is invoked per LSP by associating that LSP to a path profile policy. The path profile template is defined using NSP and requires setting the objective to use STAR WEIGHT. The path profile policy is specified with the LSP definition and is conveyed to NSP via a PCE request message.

Disjoint optimal path computation algorithm

NSP provides support for disjoint path computation between a source destination pair and between two pairs of sources and destinations. Applications can use this algorithm to provide no-impact redundancy for a service offering. The algorithm provides node/link and SRLG types of disjoint path computations. The algorithm can also re-optimize an existing path if a second path request asks to be disjoint from the existing path. The ability to treat a pair of paths as mutually disjoint requires associating a path profile ID to the path request. In addition, a path group ID specification is also essential to implicitly identify the path pair from other path pairs. The disjoint optimal path calculation algorithm can also compute paths that are bidirectionally symmetric, to ensure that forward and reverse traffic use the hops while being disjoint.

Note: NSP can only compute bidirectionally symmetric forward or reverse paths. For an RSVP LSP with primary and secondary path specification, the profile is applied to both paths. For example, if there are two RSVP LSPs between the respective distinct sources and destinations, the primary path of LSP 1 will be mutually disjoint from the primary path of LSP2, and vice versa for secondary paths. The algorithm cannot be applied to ensure the primary and secondary paths between the same source and destination pair are mutually disjoint.

Global concurrent optimization algorithm

NSP also supports optimizing the paths of existing LSPs by applying an optimization algorithm. This algorithm extracts the current resource availability on the current topology and reroutes the selected LSP paths such that the overall network consumption is minimized. The result is to utilize more network links, but also reduce the consumption on the links. LSPs must be delegated to NSP and must be preselected. Profiles do not have to be associated to the paths in order to use this algorithm. The LSPs to be optimized are selected manually using NSP's service management views.

Note: The LSPs that have a profile with the disjoint option enabled are excluded.

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