5-4
z
Short packets and long packets are fairly scheduled: if there are both long packets and short
packets in queues, statistically the short packets should be scheduled preferentially to reduce the
jitter between packets as a whole.
Compared with FQ, WFQ takes weights into account when determining the queue scheduling order.
Statistically, WFQ gives high priority traffic more scheduling opportunities than low priority traffic. WFQ
can automatically classify traffic according to the “session” information of traffic (protocol type, TCP or
UDP source/destination port numbers, source/destination IP addresses, IP precedence bits in the ToS
field, and so on), and try to provide as many queues as possible so that each traffic flow can be put into
these queues to balance the delay of every traffic flow as a whole. When dequeuing packets, WFQ
assigns the outgoing interface bandwidth to each traffic flow by precedence. The higher precedence
value a traffic flow has, the more bandwidth it gets.
Additionally, WFQ can work with the minimum guaranteed bandwidth mechanism. You can configure a
minimum guaranteed bandwidth for each WFQ queue to guarantee that each WFQ queue is
guaranteed of the bandwidth when congestion occurs. The assignable bandwidth (assignable
bandwidth = total bandwidth – the sum of the minimum guaranteed bandwidth for each queue) is
allocated to queues based on queue priority.
For example, assume that the total bandwidth of a port is 10 Mbps, and there are five flows on the port
currently, with the precedence being 0, 1, 2, 3, and 4 and the minimum guaranteed bandwidth being 128
kbps, 128 kbps, 128 kbps, 64 kbps, and 64 kbps respectively.
z
The assignable bandwidth = 10 Mbps – (128 kbps + 128 kbps + 128 kbps + 64 kbps + and 64 kbps)
= 9.5 Mbps
z
The total assignable bandwidth quota is the sum of all the (precedence value + 1)s, that is, 1 + 2 +
3 + 4 + 5 = 15.
z
The bandwidth percentage assigned to each flow is (precedence value of the flow + 1)/total
assignable bandwidth quota. The bandwidth percentages for the flows are 1/15, 2/15, 3/15, 4/15,
and 5/15 respectively.
z
The bandwidth finally assigned to a queue = the minimum guaranteed ban the bandwidth
allocated to the queue from the assignable bandwidth
Because WFQ can balance delay and jitter among flows when congestion occurs, it is effectively
applied in some special occasions. For example, WFQ is used for the assured forwarding (AF) services
of the Resource Reservation Protocol (RSVP). In Generic Traffic Shaping (GTS), WFQ is used to
schedule buffered packets.
SP+WRR queuing
By assigning some queues on the port to the SP scheduling group and the others to the WRR
scheduling group (that is, group 1), you implement SP + WRR queue scheduling on the port. Packets in
the SP scheduling group are scheduled preferentially. When the SP scheduling group is empty, packets
in the WRR scheduling group are scheduled. Queues in the SP scheduling group are scheduled with
the SP queue scheduling algorithm. Queues in the WRR scheduling group are scheduled with WRR.
Congestion Management Configuration Approaches
Complete the following tasks to achieve congestion management:
Task
Remarks
Configuring SP Queuing
Optional
Configure WRR Queuing
Optional
