Administrator Guide

● 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
● match ip vlan
By default, all packets are marked for green handling if the rate-police and trust-diffserv commands are not used in an
ingress policy map. All packets marked for red handling or “violate” are dropped.
In the class map, in addition to color-marking matching packets for yellow handling, you can also configure a DSCP value for
matching packets.
When you use ECN to classify and color-mark packets in an ingress class map, take into account:
● When all matching packets are marked for yellow treatment, policer-based coloring is not supported at the same time.
● If a single-rate two-color policer is configured at the same time as ECN-matched packets are set for yellow handling, by
default all packets less than PIR are marked for “green” handling. All green packets selected by ECN match criteria and
color-marked yellow are over-written and marked for yellow handling.
● If a two-rate three-color policer is configured at the same time as ECN-matched packets are set for yellow handling:
○ x < CIR is marked as green.
○ CIR < x< PIR is marked as yellow.
○ PIR < x is marked as red.
Green packets matching the ECN criteria for which yellow color-marking is configured are overwritten and marked as yellow.
Example: Color-marking non-ECN Packets in One Traffic Class
The following example shows how to mark non-ECN packets for “yellow” handling when all packets egress on the default queue
0. Non-ECN-capable packets have the ECN field in their packet headers set to 0.
ip access-list standard ecn_0
seq 5 permit any ecn 0
Quality of Service (QoS)
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