QoS Policing Configuration Example

Lessons Discussion
19

Abhishek,

  1. The answer this depends on whether we are talking about two color or three color. If you are talking two color with 128K or less being within policy, and the provider is allowing excess traffic but remarking it as DSCP 0, then yes, you have the ability to transmit up to whatever your physical link bandwidth would be. Of course, who knows what would happen to exceeding traffic later within the provider’s network–it might get dropped later if there is congestion elsewhere. This leads in to your question #2 …

  2. The purpose of remarking exceeding traffic as DSCP 0 is so the provider will know what traffic it can “throw away” first if the ISP network starts to get congested. Presumably, the provider has an SLA with you that says they guarantee traffic will flow through their network with a certain kind of higher level treatment for any traffic below 128 K. Once you exceed 128Kbps, however, that guarantee no longer applies. So how can the ISP distinguish between normal and exceeding traffic? That is the purpose of their policer that marks the traffic as DSCP 0.

  3. For 128K SLA with bursting to 200K, you would see something like this on the provider’s end:

R2(config)#policy-map SINGLE-RATE-THREE-COLOR
R2(config-pmap)#class class-default
R2(config-pmap-c)#police 128000 200000
R2(config-pmap-c-police)#conform-action transmit 
R2(config-pmap-c-police)#exceed-action set-dscp-transmit 0
R2(config-pmap-c-police)#violate-action drop
  1. As a customer, it is generally not a good idea to police your own traffic, since you are simply dropping it. It is better to put a shaper on the other end of a policer. So in this case, the ISP would have the policer, and you would create a shaper based on the ISP’s police values. These should match as closely as possible. With a shaper, you are queuing your own traffic, but from your application’s point of view it is probably better to delay the traffic than to lose it.

There is a lesson on shaping here: https://networklessons.com/topic/traffic-shaping-on-cisco-ios/

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