Concept Guide

Applying DSCP and VLAN Match Criteria on a Service
Queue
You can configure Layer 3 class maps which contain both a Layer 3 Differentiated Services Code Point (DSCP) and IP VLAN IDs
as match criteria to filter incoming packets on a service queue on the switch.
To configure a Layer 3 class map to classify traffic according to both an IP VLAN ID and DSCP value, use the match ip vlan
vlan-id command in class-map input configuration mode. You can include the class map in a policy map, and apply the class
and policy map to a service queue using the service-queue command. In this way, the system applies the match criteria in a
class map according to queue priority (queue numbers closer to 0 have a lower priority).
To configure IP VLAN and DSCP match criteria in a Layer 3 class map, and apply the class and policy maps to a service queue:
1. Create a match-any or a match-all Layer 3 class map, depending on whether you want the packets to meet all or any of
the match criteria. By default, a Layer 3 class map is created if you do not enter the layer2 option with the class-map
command. When you create a class map, you enter the class-map configuration mode.
CONFIGURATION mode
Dell(conf)#class-map match-all pp_classmap
2. Configure a DSCP value as a match criterion.
CLASS-MAP mode
Dell(conf-class-map)#match ipdscp 5
3. Configure an IP VLAN ID as a match criterion.
CLASS-MAP mode
Dell(conf-class-map)#match ip vlan 5
4. Create a QoS input policy.
CONFIGURATION mode
Dell(conf)#qos-policy-input pp_qospolicy
5. Configure the DSCP value to be set on matched packets.
QOS-POLICY-IN mode
Dell(conf-qos-policy-in)#set ip-dscp 5
6. Create an input policy map.
CONFIGURATION mode
Dell(conf)#policy-map-input pp_policmap
7. Create a service queue to associate the class map and QoS policy map.
POLICY-MAP mode
Dell(conf-policy-map-in)#service-queue 0 class-map pp_classmap qos-policy pp_qospolicy
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.
Quality of Service (QoS)
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