PLANET FGSW-2840 - Manual (Page 160)

User’s Manual of FGSW-2840 / FGSW-4840S
2. 802.1P Priority
Figure 4-7-2 : 802.1Q Frame
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 Managed 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 4-7-3: 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; 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 Managed Switch implements four scheduling queues, TC0, TC1, TC2 and TC3. TC0 has the lowest
priority while TC3 has the highest priority. The Managed 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 Managed 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.
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