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 Scheduler Strict.
• 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.
• Assign strict priority to one unicast queue.
CONFIGURATION mode
strict-priority
The queue range is from 1 to 7.
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 Z9500, 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 WRED.
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 example, 2000KB, is reached, all incoming packets are dropped until the buffer space
consumes less than 2000KB of the specified traffic.
738 Quality of Service (QoS)