17
3.2.1 Packet Priority Classification
Each received packet is examined and classified into one of four priority classes, Class 3, Class
2, Class 1 and Class 0 upon reception. The switch provides the following classification methods:
802.1p classification
: use User Priority tag value in the received IEEE 802.1Q packet to map to
one priority class
DSCP classification
: use DSCP value in the received IP packet to map to one priority class
Port-based classification
: used when 802.1p and DSCP are disabled or fail to be applied
They all can be configured to be activated or not. More than one classification methods can be
enabled at the same time. However, 802.1p classification is superior than DSCP classification.
802.1p mapping tables
: Each ingress port has its own mapping table for 802.1p classification.
DSCP mapping table
: All ingress ports share one DSCP mapping table for DSCP classification.
Default port priority
: A port default priority class is used when port-based classification is
applied
All configuration settings are in per port basis except that DSCP mapping table is global to all ports. A
received packet is classified into one of four priority class before it is forwarded to an egress port.
3.2.2 Priority Class Queues
Each egress port in the switch is equipped with four priority class egress queues to store the
packets for transmission. A packet is stored into the class queue which is associated to the
classified priority class. For example, a packet is stored into Class 3 egress queue if it is classified
as priority Class 3.
3.2.3 Egress Service Policy
Each port can be configured with an egress service policy to determine the transmission priority among
four class queues. By default, higher class number has higher priority than the lower class
numbers.
Four policies are provided for selection as follows:
Strict priority
: Packets in high priority class queue are sent first until the queue is empty
Weighted ratio priority Class 3:2:1:0 = 4:3:2:1
: four queues are served in 4:3:2:1 ratio
Weighted ratio priority Class 3:2:1:0 = 5:3:1:1
: four queues are served in 5:3:1:1 ratio
Weighted ratio priority Class 3:2:1:0 = 1:1:1:1
: four queues are served equally
Strict priority policy lets high priority class queue is served first until it is empty. Lower priority queue
may not get any service (or egress bandwidth) when higher priority traffic is heavy for long time. Three
weighted ratio policies are provided to resolve such problem. Four class queues are served in
weighted round robin basis. Every priority class can get a guaranteed ratio for the egress
bandwidth.
