Benefits Of Using A Configurable Weight For Wred With Ecn - Dell Z9500 Configuration Manual

Table 71. Scenarios of WRED and ECN Configuration
Queue Service-Pool
Configuration Configuration
WRED ECN WRED
0 0 X
1 0 0
1
1 1 0
1
Benefits of Using a Configurable Weight for WRED with
ECN
On the Z9500, 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.
WRED Threshold
Relationship
Q threshold = Q-T,
Service pool threshold
= SP-T
ECN
X X
X X
X Q-T < SP-T
SP-T < Q-T
X X
X Q-T < SP-T
SP-T < Q-T
Expected Functionality
WRED/ECN not applicable
Queue based WRED,
No ECN marking
SP based WRED,
No ECN marking
Queue-based ECN marking above queue threshold.
ECN marking to shared buffer limits of the service-pool
and then packets are tail dropped.
Same as above but ECN marking starts above SP-T.
Quality of Service (QoS) 687