QoS priority

[agustinmar]

Hi:

I am configuring QoS and I have a doubt about priority command. Whats is the difference between:

class REALTIME
priority
police 20000000

... and:

class REALTIME
priority 20000

Thank you.

Regards,

Agustín

[omz]

class REALTIME
priority 20000

The priority command provides a minimum bandwidth guarantee and have a built-in policer. 

https://www.cisco.com/c/en/us/support/docs/quality-of-service-qos/qos-packet-marking/10100-priorityvsbw.html

[agustinmar]

Thank you for you support, omz.

[paul driver]

Hello

Qos priority is low latency queuing (LLQ) with is a version of priority queuing (PQ) but it doesn't starve lower priority class when congestion is on the link like PQ did

Policing is more invasive and will remark or drop packets once its defied police limit has been reached as apposed to shaping which wont drop packets but will buffer them until its can send them.

[agustinmar]

Thank you!

[mlund]

Hi

From the link that OMZ provided, this is the difference

 "Any excess traffic is sent if the link is not congested or is dropped if the link is congested"

That means that from Your second example You are guaranteed 20M, but if there are unused bandwidth, You can send more traffic that is classified as priority. So if there are no other traffic than the priority You can get 100% of the bandwidth.

The first example means all traffic that is classified as priority is always prioritized, but as soon as the bandwidth goes above 20M it will be policed, and You can't get more, even if there are no other traffic.

Mikael

[Joseph W. Doherty]

The difference is much as Mikael describes, although the key, I believe, is whether there's any queuing when not using an explicit policer.

For example, as long as there's no queuing, you might have 30% LLQ and 70% other, or the converse 70% LLQ and and 30% other. Again, though, once traffic exceeds 100%, traffic will begin to queue and the LLQ's implicit policer will engage.

[agustinmar]

Thank you for all.

[agustinmar]

Many thanks