Users Guide
service-queue 2 class-map class_dscp_40
service-queue 3 class-map class_dscp_50
Using A Congurable Weight for WRED and ECN
The switch supports a user-congurable weight that determines the average queue size used in WRED and Explicit Congestion Notication
(ECN) operation on front-end I/O and backplane interfaces.
By default, the switch uses a weight factor of 0 (instantaneous ECN marking), which results in packet dropping during times of network
congestion based on the congured minimum and maximum WRED thresholds. You can congure dierent weights for WRED and ECN
operation to nely tune how dierent types of trac are handled when a WRED threshold is exceeded.
Benets of Using a Congurable Weight for WRED with ECN
Using a congurable weight for WRED and ECN allows you to specify how the average queue size is calculated. In WRED, the average
queue size determines when a threshold is exceeded and packets are dropped; in WRED with ECN, the average queue size determines
when packets are marked for later transmission and when the transmission rate is reduced on an interface during times of network
congestion.
For example, in a best-eort network topology that uses WRED with instantaneous ECN, data packets may be transmitted at a rate in
which latency or throughput are not maintained at an eective, optimal level. Packets are dropped when the network experiences a large
trac load according to the congured WRED thresholds. This best-eort network deployment is not suitable for applications that are
time-sensitive, such as video on demand (VoD) or voice over IP (VoIP) applications.
To resolve the problem of packet loss at times of network congestion, you may need to apply WRED with ECN and more nely tune packet
transmission for certain trac types. To do so, you can congure the weight used to calculate the average queue size; the average queue
size is used to determine when to drop packets with WRED and when to mark packets with ECN when WRED thresholds are exceeded.
The user-congurable weight in WRED and ECN provides better control in how the switch responds to congestion before a queue
overows and packets are dropped or delayed. Using a congurable weight for WRED and ECN allows you to customize network
performance and throughput.
Setting Average Queue Size using a Weight
You can congure the weight factor that determines the average queue size for WRED and ECN packet handling by using the wred
weight
command.
The average queue size is computed using the last calculated average-queue size and the current queue size. The following is the formula
to calculate the average queue size: average-queue-size (t+1) = average-queue-size (t) + (current-queue-length - average-queue-size
(t))/2^N
where t is the time or the current instant at which average queue size is measured, t+1 is the next calculation of the average queue size,
and N is the weight factor.
In a topology in which network congestion varies over time, you can specify a weight to enable a smooth, seamless averaging of packets to
handle the bursty nature of packets based on the previous time sampling performed. You can specify a weight value for front-end and
backplane ports separately. The range of weight values is from 0 to 15.
You can enable WRED with ECN capabilities per queue to ne-tune packet transmission. You can disable WRED with ECN per queue while
conguring the minimum and maximum buer thresholds for each WRED color-coded prole. You can congure the maximum drop-rate
percentage for yellow and green proles. You can congure these parameters for both front-end and backplane ports.
848
Quality of Service (QoS)