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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.
Dell EMC Networking OS support different types of match qualifiers to classify the incoming traffic.
Match qualifiers can be directly configured in the class-map command or it can be specified through one or more ACL which in
turn specifies the combination of match qualifiers.
Until Release 9.3(0.0), support is available for classifying traffic based on the 6-bit DSCP field of the IPv4 packet.
As a part of this feature, the 2-bit ECN field of the IPv4 packet will also be available to be configured as one of the match
qualifier. This way the entire 8-bit ToS field of the IPv4 header shall be used to classify traffic.
The Dell EMC Networking OS Release 9.3(0.0) supports the following QOS actions in the ingress policy based QOS:
1. Rate Policing
2. Queuing
3. Marking
For the L3 Routed packets, the DSCP marking is the only marking action supported in the software. As a part of this feature,
the additional marking action to set the “color” of the traffic will be provided.
Until Release 9.3(0.0), the software has the capability to qualify only on the 6-bit DSCP part of the ToS field in IPv4 Header.
You can now accept and process incoming packets based on the 2-bit ECN part of the ToS field in addition to the DSCP
categorization. The IPv4 ACLs (standard and Extended) are enhanced to add this qualifier. This new keyword ‘ecn’ is present
for all L3 ACL types (TCP/UDP/IP/ICMP) at the level where the ‘DSCP’ qualifier is positioned in the current ACL commands.
Dell EMC Networking OS supports the capability to contain DSCP and ECN classifiers simultaneously for the same ACL entry.
You can use the ecn keyword with the ip access-list standard, ip access-list extended, seq, and permit commands for standard
and extended IPv4 ACLs to match incoming packets with the specified ECN values.
Similar to ‘dscp’ qualifier in the existing L3 ACL command, the ‘ecn’ qualifier can be used along with all other supported ACL
match qualifiers such as SIP/DIP/TCP/UDP/SRC PORT/DST PORT/ ICMP.
Until Release 9.3(0.0), ACL supports classification based on the below TCP flags:
● ACK
● FIN
● SYN
● PSH
● RST
● URG
You can now use the ‘ecn’ match qualifier along with the above TCP flag for classification.
The following combination of match qualifiers is acceptable to be configured for the Dell EMC Networking OS software through
L3 ACL command:
● Classification based on DSCP only
● Classification based on ECN only
● Classification based on ECN and DSCP concurrently
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Quality of Service (QoS)