Users Guide

• 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
class-map match-any ecn_0_cmap
match ip access-group ecn_0 set-color yellow
Quality of Service (QoS) 780