Configuring Wred/Ecn - Lenovo RackSwitch G8264 Application Manual

Configuring WRED/ECN

© Copyright Lenovo 2016
WRED and ECN work together as follows:
 If the number of packets in the queue is less than the minimum threshold,
packets are transmitted. This happens irrespective of the ECN bit setting, and on
networks where only WRED (without ECN) is enabled.
If the number of packets in the queue is between the minimum threshold and

the maximum threshold, one of the following occurs:
If the ECN field on the packet indicates that the endpoints are ECN‐capable

and the WRED algorithm determines that the packet has likely been dropped
based on the drop probability, the ECT and CE bits for the packet are changed
to 1, and the packet is transmitted.
If the ECN field on the packet indicates that neither endpoint is ECN‐capable,

the packet may be dropped based on the WRED drop probability. This is true
even in cases where only WRED (without ECN) is enabled.
If the ECN field on the packet indicates that the network is experiencing

congestion, the packet is transmitted. No further marking is required.
 If the number of packets in the queue is greater than the maximum threshold,
packets are dropped based on the drop probability. This is the identical
treatment a packet receives when only WRED (without ECN) is enabled.
For configuring WRED, you must define a TCP profile and a non‐TCP profile.
WRED prioritizes TCP traffic over non‐TCP traffic.
For configuring ECN, you must define a TCP profile. You don't need a non‐TCP
profile as ECN can be enabled only for TCP traffic.
If you do not configure the profiles, the profile thresholds are set to maximum
value of 0xFFFF to avoid drops.
Note: WRED/ECN can be configured only on physical ports and not on LAG.
WRED and ECN are applicable only to unicast traffic.
Consider the following guidelines for configuring WRED/ECN:
 Profiles can be configured globally or per port. Global profiles are applicable to
all ports.
Always enable the global profile before applying the port‐level profile.

Note: You can enable the global profile and disable the port‐level profile.
However, you must not enable the port‐level profile if the global profile is disabled.
 WRED settings are dependent on Memory Management Unit (MMU) Settings. If
you change the MMU setting, it could impact WRED functionality.
You cannot enable WRED if you have QoS buffer settings such as Converged

Enhanced Ethernet (CEE), Priority‐based Flow Control (PFC), or Enhanced
Transmission Selection (ETS).
 The number of WRED profiles per‐port must match the total number of COS
Queues configured in the system.
 If you have configured a TCP profile and enabled ECN, ECN remarking happens
only if all traffic experiencing congestion is TCP traffic.
Configure a TCP profile only after enabling ECN on the interface.

 You can apply TCP and non‐TCP profile configurations irrespective of ECN
status (enabled/disabled).
Chapter 13: Quality of Service
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