when creating policy maps I wanted to understand the difference between the following statements:
police or priority
My question is what is the difference between these two commands.
police is for policing traffic that is restrain it to a certain bandwidth consumption at all times
priority is only in action when there is congestion and it tells to give priority to this traffic and police it to the value stated in the command.
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police: The router throws everything away that is above the configured rate.
priority: If there is a congestion on the interface, the router throws everything away that is above the configured rate. The traffic that stays below the configured rate gets priority.
Priority is typically used to protect important traffic like voice. Police is used to restrict the traffic that uses too much ressources and that can't be controlled in a different way.
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The priority command sends traffic to a single queue, the low latency queue (LLQ). You can have more than one class defined with priority, but what that does is rate limit each class independently as the traffic is queued into the single LLQ. As the other posters have noted, the implicit policer only engages if there's congestion. It also drops traffic, when it's active, that exceeds its rate limit.
A police statement is a rate limiter added to a policy map class; even a priority class. Unlike a priority class's implicit policer, it's always active. It's more flexible than a priority class's implicit policer as you can adjust Bc and Be, often configure average or peak measurement, and define different actions for traffic that's within rate, overrate and very much overrate (features available may vary per IOS/platform). (Note: a policer doesn't have to always discard overrate traffic, it might just mark it differently.)
Also, priority is an egress only policy feature. A policer can be use on ingress or egress policy.