Concept Guide
Using ECN, the packets are marked for transmission at a later time after the network recovers from the heavy trac state to an optimal
load. In this manner, enhanced performance and throughput are achieved. Also, the devices can respond to congestion before a queue
overows 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-congurable. You can use the wred weight number command to congure 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 buer thresholds for each color-coding of the packets. You can congure maximum drop rate
percentage of yellow and green proles. 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 congure global service pools that are shared buer pools accessed by multiple
queues when the minimum guaranteed buers for the queue are consumed. 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 conguration on the global service-pools.
You can dene WRED proles and weight on each of the global service-pools for both loss-based and lossless (PFC) service- pools. The
following events occur when you congure 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
eective based on global service-pool WRED thresholds. The queue on which the trac is scheduled must contain WRED/ECN
settings, which are enabled for WRED, to be valid for that trac.
• When WRED is congured on the global service-pool (regardless of whether ECN on global service-pool is congured), and one or
more queues have WRED enabled and ECN disabled, WRED is eective for the minimum of the thresholds between the queue
threshold and the service-pool threshold.
• When WRED is congured on the global service-pool (regardless of whether ECN on global service-pool is congured), and one or
more queues are enabled with both WRED and ECN, ECN marking takes eect. The packets are ECN marked up to shared- buer
limits as determined by the shared-ratio for that global service-pool.
WRED/ECN congurations for the queues that belong to backplane ports are common to all the backplane ports and cannot be specied
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.
The following table describes the WRED and ECN operations that occur for various scenarios of WRED and ECN conguration on the
queue and service pool. (X denotes not-applicable in the table, 1 indicates that the setting is enabled, 0 represents a disabled setting. )
Table 63. Scenarios of WRED and ECN
Conguration
Queue Conguration Service-Pool
Conguration
WRED Threshold
Relationship
Q threshold = Q-T,
Service pool threshold =
SP-T
Expected Functionality
WRED ECN WRED ECN
0 0 X X X WRED/ECN not applicable
Quality of Service (QoS) 699