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Explicit Congestion Notification
Explicit Congestion Notification (ECN) enhances and extends WRED functionality by marking packets for
later transmission instead of dropping them when a threshold value is exceeded. Use ECN for WRED to
reduce the packet transmission rate in a congested, heavily-loaded network.
While WRED drops packets to indicate congestion, ECN marks packets instead of dropping them when
the average queue length exceeds the threshold value. ECN provides an improved method for
congestion avoidance by allowing the switch to mark packets for later transmission rather than dropping
them from a queue.
ECN uses a two-bit ECN-specific field in the IP header to indicate if a packet is ECN-capable, if the
endpoints in the transport protocol are ECN-capable, and if there is network congestion.
When ECN for WRED is enabled, if the queue length is between the minimum threshold and the
maximum threshold, one of the following actions is taken:
• If the WRED drop precedence determines that the packet should be dropped but the ECN field in the
packet header indicates that the endpoints are ECN-capable, the packet is marked with a congestion-
experienced (CE) bit and transmitted.
• If the ECN field indicates that both endpoints are not ECN-capable, the packet can be dropped
according to the configured WRED drop precedence.
• If the ECN field indicates a network congestion condition, the packet is marked with a congestion-
experienced (CE) bit and then transmitted.
If the queue length falls below the minimum threshold or exceeds the maximum threshold, the same
WRED treatment is applied as when ECN is not enabled:
• If queued packets fall below the minimum threshold, they are transmitted.
• If queued packets exceed the maximum threshold, they are dropped.
ECN Packet Classification
When ECN for WRED is enabled on an interface, non-ECN-capable packets are marked as green-profiled
traffic and are subject to early WRED drops. For example, TCP-acks, OAM, and ICMP ping packets are
non-ECN-capable. However, it is not desirable for these packets to be WRED-dropped. You can use ECN
match criteria in an ingress class map or an ACL to classify ECN-capable and non-ECN-capable packets
and apply the appropriate color-based WRED action.
Standard and extended IPv4 ACLs support the use of the 2-bit ECN field in packet headers as L3 deny/
permit criteria for IP, TCP, UDP, and ICMP packets. Enter the keyword ecn in a deny/permit statement to
mark ingress traffic according to its ECN-capability or non-capability. You can specify DSCP and ECN
classifiers in the same ACL entry in an IP standard or extended ACL.
In a match-any class map, you can mark selected ECN/non-ECN traffic for yellow handling by entering
set-color yellow in any of the following L3 match commands:
• match ip access-group
• match ip dscp
• match ip precedence
Quality of Service (QoS)
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