Loading ...
Loading ...
Loading ...
2. 802.1P Priority
Figure 9-2 802.1Q 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 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; 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
113
Loading ...
Loading ...
Loading ...