Path Selection Tie-Breaking
If more than one path is available after applying the rules from the previous section, a tie-breaking rule is applied to choose the path for the LSP. There are three tie-breaking rules: random, least fill, and most fill. The rule used depends on the configuration. Random is the default rule. For other rules, the following definitions are needed:
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
- 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.
- 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.