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3. DSCP based
Figure 8-4 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 priority in the range of 0 to 63. The last 2 bits (bit 6 and bit 7) are reserved. The
mapping relationship between sixty-four DSCP priority value and priority queues is shown as
follows:
Priority 0 to 15 are assigned to the 1 (Lowest) priority queue.
Priority 16 to 31 are assigned to the 2 (Normal) priority queue.
Priority 32 to 47 are assigned to the 3 (Medium) priority queue.
Priority 48 to 63 are assigned to the 4 (Highest) priority queue.
When DSCP QoS mode is enabled, the switch will automatically map the ingress packets to
priority queues based on the DSCP precedence and the above mapping relationship. After that,
packets in all the queues are sent in order based on the weight value for each queue. The
switch implements four scheduling queues, TC1, TC2, TC3 and TC4. TC1 has the lowest priority
while TC4 has the highest priority. The weight value ratio of TC1, TC2, TC3 and TC4 is 1:2:4:8.
As for the untagged packets, the switch will forward it according to the default priority mode.
The QoS module is mainly for priority configuration and traffic control, including three
submenus: QoS Basic, Bandwidth Control and Storm Control.
8.1 QoS Basic
This switch classifies the ingress packets, maps the packets to different priority queues and
then forwards the packets to implement QoS function.
This switch implements three priority modes based on port /802.1P /DSCP. The port-based QoS
mode supports four priority queues. The port priority queues are labeled as 1, 2, 3, and 4.
On this page you can configure and view QoS mode and the port-based priority setting.
Choose the menu QoS→QoS Basic to load the following page.
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