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Marking Using Policers

Traffic policers measure the traffic rate for data entering or exiting an interface, with the goal of determining if a configured traffic contract has been exceeded. The contract has two components: a traffic rate, configured in bits/second, and a burst size, configured as a number of bytes. If the traffic is within the contract, all packets are considered to have conformed to the contract.

However, if the rate or burst exceeds the contract, then some packets are considered to have exceeded the contract. QoS actions can be taken on both categories of traffic.

The simplest form of policing enforces the traffic contract strictly by forwarding conforming packets and discarding packets that exceed the contract. However, both IOS policers allow a compromise action in which the policer marks down packets instead of dropping them. To mark down the packet, the policer re-marks a QoS field, typically IPP or DSCP, with a value that makes the packet more likely to be discarded downstream. For instance, a policer could re-mark AF11 packets that exceed a contract with a new DSCP value of AF13, but not discard the packet. By doing so, the packet still passes through the router, but if the packet experiences congestion later in its travels, it is more likely to be discarded than it would have otherwise been. (Remember, DiffServ suggests that AF13 is more likely to be discarded than AF11 traffic.)

When marking requirements can be performed by using CB Marking, CB Marking should be used instead of either policer. However, if a requirement exists to mark packets based on whether they conform to a traffic contract, marking with policers must be used. Chapter 16,

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