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3-17. QoS(Quality of Service) Configuration
The switch supports 5 kinds of QoS, are as follows, MAC Priority, 802.1p Priority, IP
TOS Priority, and DiffServ DSCP Priority. Port Based Priority has a special name
called VIP Port in the switch. Any packets enter VIP Port will have highest
transmitting priority. MAC Priority act on the destination address of MAC in packets.
VLAN tagged Priority field is affected by 802.1p Priority setting. IP TOS Priority
affects TOS fields of IP header, and you can find it has 8-bit SERVICE TYPE field
that specifies how the datagram should be handled. The field could be divided into
six subfields as follows, PRECEDENCE (3 bits), D-Type (Delay Priority, 1 bit), T-
Type (Throughput Priority, 1bit), R-Type (Reliability Priority, 1bit), M-Type (Monetary
Cost Priority, 1bit), and UNUSED (1bit).
User can randomly control these fields to achieve some special QoS goals. When
bits D, T, R, or M set, the D bit requests low delay, the T bit requests high
throughput, the R bit requests high reliability, and the M bit requests low cost.
DiffServ DSCP Priority act on DSCP field of IP Header. In the late 1990s, the IETF
redefined the meaning of the 8-bit SERVICE TYPE field to accommodate a set of
differentiated services (DS). Under the differentiated services interpretation, the first
six bits comprise a codepoint, which is sometimes abbreviated DSCP, and the last
two bits are left unused.
High Priority Packet streams will experience less delay into the switch. For handing
different priority packets, each egress port has designed up to 4 queues. Each QoS
is influenced by two scheduling, WRR (Weighted Round Robin) and Strict Priority as
well. When you finish setting the priority mapping to the queue, WRR scheduling will
distribute the bandwidth according to the weight you set for 4 queues (queue 0 to
queue 3). Another scheduling is Strict Priority dedicated for the function named VIP
Port of QoS. While we select some ports as the VIP Port, these ports will own the
highest transmitting priority in egress queue of the switch.
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