Enabling Strict-Priority Queueing; Weighted Random Early Detection - Dell C9000 Series Networking Configuration Manual

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 rate-shaping as well as strict-priority queuing at the same time on a
queue, use the Scheduler Strict feature as described in
• The strict-priority supersedes bandwidth-percentage and bandwidth-weight percentage
configurations.
• A queue with strict priority can starve other queues in the same port-pipe.
NOTE: Assigning strict priority scheduling to a unicast queue on all ports using a global command is
not supported. However, you can configure both unicast and multicast queue belonging to a dot1p
to use strict priority scheduling using policy maps and then associate the policy map to the egress
interface.

Weighted Random Early Detection

Weighted random early detection (WRED) is a congestion avoidance mechanism that drops packets to
prevent buffering resources from being consumed.
NOTE: On the switch, WRED and Explicit Congestion Notification (ECN) marking are supported on front-
end I/O and backplane HiGig ports. When you enable WRED, packets are dropped during times of
network congestion based on the configured minimum and maximum WRED thresholds. ECN marks
packets for later transmission (instead of dropping them) when the network recovers from a heavy traffic
condition. For information about how to configure weights for WRED and ECN operation, see
Configuring Weights and ECN for
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, for
Scheduler
WRED.
Strict.
Quality of Service (QoS) 926