Path Selection Tie-Breaking
If more than one path is available after the rules from the previous section have been applied, a tie-breaking rule is applied to choose the path for the LSP. 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.
- 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 rule used depends on the configuration. Random is the default rule.
For the 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
