Users Guide

space on the buer and trac manager (BTM) (ingress or egress) can be consumed by only one or few types of trac, leaving no
space for other types. You can apply a WRED prole to a policy-map so that the specied trac can be prevented from consuming
too much of the BTM resources.
WRED drops packets when the average queue length exceeds the congured threshold value to signify congestion. ECN is a
capability that enhances WRED by marking the packets instead of causing WRED to drop them when the threshold value is
exceeded. If you congure ECN for WRED, devices employ ECN to mark the packets and reduce the rate of sending packets in a
congested network.
In a best-eort network topology, data packets are transmitted in a manner in which latency or throughput is not maintained to be at
an eective level. Packets are dropped when the network experiences a large trac load. This best-eort network deployment is not
suitable for applications that are time-sensitive, such as video on demand (VoD) or voice over IP (VoIP) applications. In such cases,
you can use ECN in conjunction with WRED to resolve the dropping of packets under congested conditions.
Using ECN, the packets are marked for transmission at a later time after the network recovers from the heavy trac state to an
optimal load. In this manner, enhanced performance and throughput are achieved. Also, the devices can respond to congestion
before a queue overows and packets are dropped, enabling improved queue management.
When a packet reaches the device with ECN enabled for WRED, the average queue size is computed. To measure the average
queue size, a weight factor is used. This weight factor is user-congurable. You can use the wred weight number command to
congure the weight for the WRED average queue size. The mark probability value is the number of packets dropped when the
average queue size reaches the maximum threshold value.
The weight factor is set to zero by default, which causes the same behavior as dropping of packets by WRED during network loads
or also called instantaneous ECN marking. In a topology in which congestion of the network varies over time, you can specify a
weight to enable a smooth, seamless averaging of packets to handle the sudden overload of packets based on the previous time
sampling performed. You can specify the weight parameter for front-end and backplane ports separately in the range of 0 through
15.
You can enable WRED and ECN capabilities per queue for granularity. You can disable these functionality per queue, and you can also
specify the minimum and maximum buer thresholds for each color-coding of the packets. You can congure maximum drop rate
percentage of yellow and green proles. You can set up these parameters for both front-end and backplane ports.
Global Service Pools With WRED and ECN Settings
Support for global service pools is now available. You can congure global service pools that are shared buer pools accessed by
multiple queues when the minimum guaranteed buers for the queue are consumed. S4820T platform support four global service-
pools in the egress direction. Two service pools are used– one for loss-based queues and the other for lossless (priority-based ow
control (PFC)) queues. You can enable WRED and ECN conguration on the global service-pools.
You can dene WRED proles and weight on each of the global service-pools for both loss-based and lossless (PFC) service- pools.
The following events occur when you congure WRED and ECN on global service-pools:
• If WRED/ECN is enabled on the global service-pool with threshold values and if it is not enabled on the queues, WRED/ECN are
not eective based on global service-pool WRED thresholds. The queue on which the trac is scheduled must contain
WRED/ECN settings, which are enabled for WRED, to be valid for that trac.
• When WRED is congured on the global service-pool (regardless of whether ECN on global service-pool is congured), and one
or more queues have WRED enabled and ECN disabled, WRED is eective for the minimum of the thresholds between the
queue threshold and the service-pool threshold.
• When WRED is congured on the global service-pool (regardless of whether ECN on global service-pool is congured), and one
or more queues are enabled with both WRED and ECN, ECN marking takes eect. The packets are ECN marked up to shared-
buer limits as determined by the shared-ratio for that global service-pool.
WRED/ECN congurations for the queues that belong to backplane ports are common to all the backplane ports and cannot be
specied separately for each backplane port granularity. This behavior occurs to prevent system-level complexities in enabling this
support for backplane ports. Also, WRED/ECN is not supported for multicast packets.
Quality of Service (QoS)
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