Supported Platforms
Path Selection Tie-Breaking
If more than one path is still available after the CSPF rules (How CSPF Selects a Path) have been applied, a tie-breaking rule is applied to choose the path for the LSP. The rule used depends on the configuration. There are three tie-breaking rules:
- Random—One of the remaining paths is picked at random. This rule tends to place an equal number of LSPs on each link, regardless of the available bandwidth ratio. This is the default behavior.
- Least fill—The path with the largest minimum available bandwidth ratio is preferred. This rule tries to equalize the reservation on each link.
- Most fill—The path with the smallest minimum available bandwidth ratio is preferred. This rule tries to fill a link before moving on to alternative links.
The following definitions describe how a figure for minimum available bandwidth ratio is derived for the least fill and most fill rules:
- Reservable bandwidth = bandwidth of link x subscription factor of link
- Available bandwidth = reservable bandwidth – (sum of the bandwidths of the LSPs traversing the link)
- Available bandwidth ratio = available bandwidth/reservable bandwidth
- Minimum available bandwidth ratio (for a path) = the smallest available bandwidth ratio of the links in a path
 | Note: For the least fill or most fill behaviors to be used, the paths must have their bandwidth (specified using the bandwidth statement at the [edit protocols mpls label-switched-path lsp-name] hierarchy level) or minimum bandwidth (specified using the minimum-bandwidth statement at the [edit protocols mpls label-switched-path lsp-name auto-bandwidth] hierarchy level) configured to a value greater than 0. If the bandwidth or minimum bandwidth for the paths is either not configured or configured as 0, the minimum available bandwidth cannot be calculated and the random path selection behavior is used instead. |
