|
|
|
Links with a sufficiently low (good) performance score (an CNA-calculated metric based on network quantities and penalties, described in the section Metric) make up a winner set. These links are then considered candidates for assertion as best performance link for the prefix.
The low end of the winner-set range is the lowest current performance score among all of the links that can route traffic to the prefix.
The high end of the range is the low-end number plus a user-configurable winner-set-width value. (See the winner-set-width command in the CNA Command Reference Guide. The command takes two values. The first determines the width of the winner set; the second determines the width of the set that is used in the load optimization algorithm. See Extended Winner Set later in this chapter.)
For example, if the lowest performance metric among your service provider links is 300 for a particular prefix, and you have set the winner-set width at 25, then any link with a penalty-adjusted metric that is 325 or lower is considered to be in the winner set for that prefix.
If the current winner--the link that is currently carrying traffic to the prefix--is in the winner set, then the CNA system will take no action, even if the link currently carrying the traffic does not have the lowest metric.
This restriction is intended to prevent the CNA system from making changes to a stable network for negligible gains. This behavior can be controlled with the damped-mode command. This behavior occurs when damped-mode is enabled, which is the default value. Disabling damped-mode allows the CNA system to pick the preferred link as a winner as soon as it comes back into the winner set.
If the current winner is not in the winner set, but the set contains the BGP-selected route from each of your peerings, then the CNA decision maker process will withdraw its assertion and let BGP make the selection.
If the winner set does not contain the current winner, then the CNA system will continue to narrow the field, first by throwing out all links whose priority is less than the maximum priority in the set. Priority is determined by the winner-set-priority command.
If more than one link remains in the set, and you have enabled either link-load, prefer-in-winner-set most-reliable or prefer-in-winner-set bgp selection criteria, the decision maker will first evaluate link load, then, if a choice still needs to be made, most-reliable links will be given preference, and then, if a choice still needs to be made, BGP-selected links will be given preference.
The last step, if the winner set still contains more than one link, is random selection. The decision maker process will select one of the remaining routes and assert it as the new winner.
Use the show statistics dmaker command (or view engine statistics in the web interface) to see the current width.
|
|
|