Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.100.0 - Page 692

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. Dell 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 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. 692 Quality of Service (QoS)