Huawei Quidway S3100 Series Operation Manual page 304

Operation Manual – QoS
Quidway S3100 Series Ethernet Switches
As shown in Figure 1-2, an 802.1Q-enabled host will include a 4-bit 802.1Q header
next to the source address in its outbound Ethernet frames. The 4-bit header is
comprised of a 2-bit Tag Protocol Identifier (TPID) with a value of 8100 and a 2-bit Tag
Control Information (TCI). TPID was defined recently by IEEE, indicating that the
packet includes an 802.1Q tag. The detailed make-up of an 802.1Q header is illustrated
in Figure 1-3.
Figure 1-3 802.1Q tag header
In Figure 1-3, the Priority field in TCI is 802.1p priority. It consists of three bits with a
value range of 0 to 7. The three bits are intended to determine the priority of a given
packet. A total of 8 priorities are provided to help the switch decide which packet to be
sent first in case of traffic congestion.
As it is the 802.1p specifications that have described in detail the application of these
priorities, hence the name. It is also known as CoS priority.
III. Queue scheduling
When congestion occurs, packets contend for network resources. To address this issue,
queue scheduling was introduced. The following are two ways of implementing it:
Weighted Round Robin (WRR) and High Queue-WRR (HQ-WRR).
1) WRR
With each port supporting four output queues, WRR performs a round-robin scheduling
to ensure that all queues are served. Each queue is allocated with a weight value (w3,
w2, w1, and w0) denoting the resource proportions allocated to the corresponding
queue. For example, on a 100M port, allocating its output queues with WRR weight
values of 50, 30, 10, and 10 (corresponding to w3, w2, w1,and w0 respectively) will
ensure that even queues with the lowest priority can get at least 10Mbit/s bandwidth,
thus avoiding the scenario where packets with low priority are not served over a long
time. Performing round-robin scheduling, WRR also ensures that network bandwidth is
used efficiently by not allocating a fixed amount of time to each individual queue. This
means that when a queue has nothing to output, its bandwidth can be given to the next
queue with outputs.
2) HQ-WRR
HQ-WRR is an improvement over WRR. With queue 3 allocated with the highest priority,
the switch will ensure that this queue get served first and will perform round-robin
scheduling to the other three queues when the traffic has exceeded the bandwidth
capacity of a port.
Huawei Technologies Proprietary
1-2
Chapter 1 QoS Configuration