Understanding Tricolor Marking Architecture

Tricolor marking (TCM) policers provide two functions: metering and marking. A policer meters each packet and passes the packet and the metering result to the marker.

The meter operates in two modes. In the color-blind mode, the meter treats the packet stream as uncolored. Any preset loss priorities are ignored. In the color-aware mode, the meter inspects the packet loss priority (PLP) field, which has been set by an upstream device as high or low; in other words, the PLP field has already been set by a behavior aggregate (BA) or multifield (MF) classifier. The marker changes the PLP of each incoming IP packet according to the results of the meter.

Single-rate TCM is so called because traffic is policed according to one rate—the committed burst rate (CBR)—and two burst sizes: the committed burst size (CBS) and the excess burst size (EBS). The configured information rate (CIR) specifies the average rate at which bits are admitted to the network. The CBS specifies the usual burst size in bytes and the EBS specifies the maximum burst size in bytes for packets that are admitted to the network. The EBS is greater than or equal to the CBS, and neither can be 0. As each packet enters the network, its bytes are counted. Packets that do not exceed the CBS are marked low PLP. Packets that exceed the peak information rate (PIR) are marked high PLP.

Two-rate TCM is so called because traffic is policed according to two rates: the CIR and the PIR. The PIR is greater than or equal to the CIR. The CIR specifies the average rate at which bits are admitted to the network, and the PIR specifies the maximum rate at which bits are admitted to the network. As each packet enters the network, its bits are counted. Bits in packets that do not exceed the CIR have their packets marked low PLP. Bits in packets that exceed the PIR have their packets marked high PLP.