Single-rate QoS defines a single rate for traffic that is subject to QoS. Single-rate QoS is the most basic form of rate limiting and is well suited for constant rate traffic flows such as video or where more complex dual-rate QoS is not needed. The traffic that meets the rate requirement is considered in-profile. Traffic that does not meet the specified rate is considered out-of-profile. A two-color system is often used to describe or mark the single-rate QoS result. In-profile traffic is marked green, and out-of-profile traffic is marked red.
Single-rate rate-limiters pass traffic that is in-profile or marked green. Out-of-profile traffic (marked red) is subject to whatever action is configured for out-of-profile traffic. Out of profile traffic can be forwarded if bandwidth is available, dropped, or marked for a possible drop later in the communication path.
All traffic that arrives at or below the PR is considered in-profile and marked green. All traffic that arrives above the PR is considered out-of-profile and marked red. When the traffic exceeds the capacity of the rate-limiting component, all traffic that is marked red is dropped.
Another type of single-rate QoS is used on SummitStack and ExtremeSwitching series switches. A committed information rate (CIR) establishes a reserved traffic rate, and a peak burst size (PBS) establishes a maximum size for a traffic stream. If a traffic stream is at or below the CIR and the PBS, it is considered to be within profile and marked green. If a traffic stream exceeds either the CIR or the PBS, it is considered out-of-profile and marked red. On these switches, you can configure the single-rate rate-limiting components to drop traffic marked red or set a drop precedence for that traffic. You can also specify a DSCP value to mark the out-of-profile traffic.