Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.100.0 - Page 684

• Preamble: 7 bytes Preamble • Start frame delimiter (SFD): 1 byte • Destination MAC address: 6 bytes • Source MAC address: 6 bytes • Ethernet Type/Length: 2 bytes • Payload: (variable) • Cyclic redundancy check (CRC): 4 bytes • Inter-frame gap (IFG): (variable) You can optionally include overhead fields in rate metering calculations by enabling QoS rate adjustment. QoS rate adjustment is disabled by default. • Specify the number of bytes of packet overhead to include in rate limiting, policing, and shaping calculations. CONFIGURATION mode qos-rate-adjust overhead-bytes For example, to include the Preamble and SFD, type qos-rate-adjust 8. For variable length overhead fields, know the number of bytes you want to include. The default is disabled. The range is from 1 to 31. Enabling Strict-Priority Queueing In strict-priority queuing, the system de-queues all packets from the assigned queue before servicing any other queues. You can assign strict-priority to one unicast queue, using the strict-priority command. • Policy-based per-queue rate shaping is not supported on the queue configured for strict-priority queuing. To use queue-based rateshaping as well as strict-priority queuing at the same time on a queue, use the Scheduler Strict feature as described in Scheduler Strict . • The strict-priority supersedes bandwidth-percentage configuration. • A queue with strict priority can starve other queues in the same port-pipe. • Assign strict priority to one unicast queue. CONFIGURATION mode strict-priority The range is from 1 to 3. Weighted Random Early Detection Weighted random early detection (WRED) is a congestion avoidance mechanism that drops packets to prevent buffering resources from being consumed. The WRED congestion avoidance mechanism drops packets to prevent buffering resources from being consumed. Traffic is a mixture of various kinds of packets. The rate at which some types of packets arrive might be greater than others. In this case, the space on the buffer and traffic manager (BTM) (ingress or egress) can be consumed by only one or a few types of traffic, leaving no space for other types. You can apply a WRED profile to a policy-map so that specified traffic can be prevented from consuming too much of the BTM resources. WRED uses a profile to specify minimum and maximum threshold values. The minimum threshold is the allotted buffer space for specified traffic, for example, 1000KB on egress. If the 1000KB is consumed, packets are dropped randomly at an exponential rate until the maximum threshold is reached (as shown in the following illustration); this procedure is the "early detection" part of WRED. If the maximum threshold, 684 Quality of Service (QoS)