As
shown in the figure above, each 802.1Q Tag has a Pri field, comprising 3 bits. The 3-bit priority
field is 802.1p priority in the range of 0 to 7. 802.1P priority determines the priority of the packets
based on the Pri value. On the Web management page of the switch, you can configure different
priority tags mapping to the corresponding priority levels, and then the switch determine which
packet is sent preferentially when forwarding packets. The switch processes untagged packets
based on the default priority mode.
3. DSCP Priority
Figure 9-3 IP datagram
As shown in the figure above, the ToS (Type of Service) in an IP header contains 8 bits. The first
three bits indicate IP precedence in the range of 0 to 7. RFC2474 re-defines the ToS field in the IP
packet header, which is called the DS field. The first six bits (bit 0-bit 5) of the DS field indicate
DSCP precedence in the range of 0 to 63. The last 2 bits (bit 6 and bit 7) are reserved. On the Web
management page, you can configure different DS field mapping to the corresponding priority
levels. Non-IP datagram with 802.1Q tag are mapped to different priority levels based on 802.1P
priority mode if 8021.1P Priority mode is enabled; the untagged non-IP datagram are mapped
based on port priority mode.
Schedule Mode
When the network is congested, the problem that many packets compete for resources must be
solved, usually in the way of queue scheduling. The switch implements four scheduling queues,
TC0, TC1, TC2 and TC3. TC0 has the lowest priority while TC3 has the highest priority. The switch
provides four schedule modes: SP, WRR, SP+WRR and Equ.
1. SP-Mode: Strict-Priority Mode. In this mode, the queue with higher priority will occupy the
whole bandwidth. Packets in the queue with lower priority are sent only when the queue with
higher priority is empty. The switch has four egress queues labeled as TC0, TC1, TC2 and
TC3. In SP mode, their priorities increase in order. TC3 has the highest priority. The
disadvantage of SP queue is that: if there are packets in the queues with higher priority for a
long time in congestion, the packets in the queues with lower priority will be “starved to death”
because they are not served.
Figure 9-4 SP-Mode
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