Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 782

policy-map-input pmap_dscp_40_50 service-queue 2 class-map class_dscp_40 service-queue 3 class-map class_dscp_50 Using A Configurable Weight for WRED and ECN The switch supports a user-configurable weight that determines the average queue size used in WRED and Explicit Congestion Notification (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 configured minimum and maximum WRED thresholds. You can configure different weights for WRED and ECN operation to finely tune how different types of traffic are handled when a WRED threshold is exceeded. Benefits of Using a Configurable Weight for WRED with ECN Using a configurable 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-effort 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 effective, optimal level. Packets are dropped when the network experiences a large traffic load according to the configured WRED thresholds. This best-effort 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 finely tune packet transmission for certain traffic types. To do so, you can configure 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-configurable weight in WRED and ECN provides better control in how the switch responds to congestion before a queue overflows and packets are dropped or delayed. Using a configurable weight for WRED and ECN allows you to customize network performance and throughput. Setting Average Queue Size using a Weight You can configure 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 fine-tune packet transmission. You can disable WRED with ECN per queue while configuring the minimum and maximum buffer thresholds for each WRED color-coded profile. You can configure Quality of Service (QoS) 782