Policers and shapers identify traffic violations in an identical manner, but treat them differently. Policers perform instantaneous checks and immediately take action when a violation occurs. Actions can include marking, dropping, and even just transmitting the packet. Shapers on the other hand are traffic-smoothing tools. Its objective is to send all traffic out a given interface, but to smooth it out so that it never exceeds a given rate – usually in order to meet SLAs. Excess traffic is buffered and delayed until the traffic once again dips below the defined maximum rate.
| Policer | Shaper |
| Causes TCP resends as traffic is dropped | Delays traffic; involves less TCP resends |
| Inflexible; makes instant drop decisions | Adapts to network congestion by queuing excess traffic |
| Ingress or egress interface tool | Typically egress only |
| Rate limiting – no buffering | Rate limiting with buffering |
While policing and shaping tools are not employed to directly provide QoS for real-time traffic, they do regulate/stabilize traffic flows so that unexpected bursts in data traffic do not induce jitter and latency that adversely affects real-time traffic.
Policers determine whether each packet conforms, exceeds, or violates the policies configured for traffic, and takes the prescribed action in each case.
- Conforming – traffic that falls within the rate configured for the policer
- Exceeding – traffic that is above the policer rate, but still within the burst parameters
- Violating – traffic that is above both the policer rate and burst parameters
It is not productive to police voice traffic or call-signaling traffic because the incoming rates of these traffic types should be controlled at their origin by call admission control (CAC) mechanisms.
You can also use a policer as a marker to re-mark traffic upon an exceed and/or violate action rather than just drop it.
Although a policer can be deployed ingress or egress, it is typically deployed at the network edge on traffic ingress. If packets will be dropped, there is little point in spending CPU cycles routing these packets. Policers are also often deployed at the traffic egress interface to control bandwidth used or allocated to a particular class of traffic.
As mentioned earlier, shapers are similar to policers in that they also limit the transmission rate of packets but they do so by delaying packets that exceed the CIR. This allows for conformance to SLAs. Shaping is crucial for non-broadcast multi-access (NBMA) topologies such as ATM and Frame Relay, or potentially anywhere else where a speed mismatch may exist. Examples of this would be line speed mismatches, aggregated traffic oversubscription, and SLA enforcement by a carrier.
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