Dell S4820T, Configuration Manual

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But ‘Green’ packets matching the specific match criteria for which ‘color-marking’ is configured will be
over-written and marked as “Yellow”.
Sample configuration to mark non-ecn packets as “yellow” with Multiple
traffic class
Consider the example where there are no different traffic classes that is all the packets are egressing on
the default ‘queue0’.
Dell Networking OS can be configured as below to mark the non-ecn packets as yellow packets.
!
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
!
policy-map-input ecn_0_pmap
service-queue 0 class-map ecn_0_cmap
Applying this policy-map “ecn_0_pmap” will mark all the packets with ‘ecn == 0’ as yellow packets on
queue0 (default queue).
Classifying Incoming Packets Using ECN and Color-Marking
Explicit Congestion Notification (ECN) is a capability that enhances WRED by marking the packets instead
of causing WRED to drop them when the threshold value is exceeded. If you configure ECN for WRED,
devices employ this functionality of ECN to mark the packets and reduce the rate of sending packets in a
congested, heavily-loaded network.
ECN is a mechanism using which network switches indicate congestion to end hosts for initiating
appropriate action. End hosts uses two least significant bits of ToS to indicate that it is ECT. When
intermediate network node encounters congestion, remarks ECT to CE for end host to take appropriate
action. During congestion, ECN enabled packets are not subject to any kind of drops like WRED except
tail drops. Though ECN & WRED are independent technologies, BRCM has made WRED a mandatory for
ECN to work.
On ECN deployment, the non-ECN packets that are transmitted on the ECN-WRED enabled interface will
be considered as Green packets and will be subject to the early WRED drops. Typically the TCP-acks,
OAM, ICMP ping packets will be non-ECN in nature and it is not desirable for this packets getting WRED
dropped.
In such a condition, it is necessary that the switch is capable to take differentiated actions for ECN/Non-
ECN packets. After classifying packets to ECN/Non-ECN, marking ECN and Non-ECN packets to different
color packets is performed.
Policy based ingress QOS involves the following three steps to achieve QOS:
1. Classification of incoming traffic.
2. Specify the differentiated actions for different traffic class.
3. Attach the policy-map to the interface.
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Quality of Service (QoS)