500 BigIron RX Series Configuration Guide53-1001986-01Traffic reduction parameters and algorithm19The requested rate represents a percentage of an interface's line rate (bandwidth), expressed inbits per second (bps).Requested Rate must be entered in multiples of 515,624 bps. If you enter a number that is not amultiple of 515,624, the software adjusts the rate down to the lowest multiple of the number sothat the calculation of credits does not result in a remainder of a partial Credit. For example, if youenter 600,000 bps, the value will be adjusted to 515,624 bps.Maximum burstMaximum burst provides a higher than requested rate to traffic that meet the rate limiting criteria.When the traffic on the port is less than the specified requested rate, the rate limiting policy canaccumulate credits up to a maximum, as specified in the maximum burst value. The accumulatedcredit allows traffic to pass through the port for a short period of time, at a rate higher than theaverage rate. The time period is determined by the amount of credit accumulated and the rate oftraffic passing through the port.The maximum burst rate cannot be smaller than 65536 bitsActual rateThe device determines actual rate limiting rates through the use of proprietary formulas built intothe packet processor hardware. The resulting rate that is the closest to the requested rate. Thisleads to variable rate limiting granularities for rate limiting rates. The lower the configured ratelimiting rate, the finer the granularity; the higher the configured rate limiting rate, the larger the rateincrements. For example, at lower rates, say from 20,345 to 40, 330 bps, the configurable ratelimiting rates can increment by 1 bps, but at higher rate limiting rates, say between 4 Gbps toline-rate, the configurable rates increment in the hundreds of Mbps.The CLI shows actual rate as requested rate.Credits and credit totalEach rate limiting policy is assigned a class. A class uses the average rate and maximum allowedburst in the rate limiting policy to calculate credits and credit totals.Credit size is measured in bytes. A credit is a forwarding allowance for a rate-limited port, and is thesmallest number of bytes that can be allowed during a rate limiting interval. The minimum creditsize can be 1 byte.During a rate limiting interval, a port can send or receive only as many bytes as the port has Creditsfor. For example, if an inbound rate limiting policy results in a port receiving two credits per ratelimiting interval, the port can send or receive a maximum of 2 bytes of data during that interval.The credit size is calculated using the following algorithm.Credit = (Average rate in bits per second)/(8*64453)One second is divided into 64,453 intervals. In each interval, the number of bytes equal to thecredit size is added to the running total of the class. The running total of a class represents thenumber of bytes that can be allowed to pass through without being subject to rate limiting.The second parameter is the maximum credit total, which is also measured in bytes. The maximumcredit total is calculated using the following algorithm.Maximum credit total = (Maximum burst in bits)/8
