Administrator Guide

3 0
4 0
5 0
6 0
7 0
DELL#show qos statistics wred-profile peGigE 0/1/1
Interface peGigE 0/1/1
Drop-statistic Dropped Pkts
Green 0
Yellow 0
Out of Profile 0
Displaying egress-queue Statistics
To display the number of transmitted and dropped packets on the egress queues of a WRED-configured interface, use the following
command.
• Display the number of packets and number of bytes on the egress-queue profile.
EXEC Privilege mode
show qos statistics egress-queue
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
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Quality of Service (QoS)