Guidelines for Configuring ECN for Classifying and Color-MarkingPacketsKeep the following points in mind while configuring the marking and mapping of incoming packets using ECN fields in IPv4 headers:• Currently Dell Networking OS supports matching only the following TCP flags:– ACK– FIN– SYN– PSH– RST– URGIn the existing software, ECE/CWR TCP flag qualifiers are not supported.• Because this functionality forcibly marks all the packets matching the specific match criteria as ‘yellow’, Dell Networking OS doesnot support Policer based coloring and this feature concurrently.• If single rate two color policer is configured along with this feature, then by default all packets less than PIR would be consideredas “Green” But ‘Green’ packets matching the specific match criteria for which ‘color-marking’ is configured will be over-writtenand marked as “Yellow”.• If two rate three color policer is configured along with this feature then,– x < CIR – will be marked as “Green”– CIR < x< PIR – will be marked as “Yellow”– PIR < x – will be marked as “Red”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 classConsider 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_0seq 5 permit any ecn 0class-map match-any ecn_0_cmapmatch ip access-group ecn_0 set-color yellow!policy-map-input ecn_0_pmapservice-queue 0 class-map ecn_0_cmapApplying 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-MarkingExplicit Congestion Notification (ECN) is a capability that enhances WRED by marking the packets instead of causing WRED to dropthem when the threshold value is exceeded. If you configure ECN for WRED, devices employ this functionality of ECN to mark thepackets 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 usestwo least significant bits of ToS to indicate that it is ECT. When intermediate network node encounters congestion, remarks ECT to598 Quality of Service (QoS)
