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
Port Based QoS
- Assign priority to packets based on the incoming port on
the Switch.

DSCP Based QoS - Assign priority to packets based on their Differentiated
Services Code Points (DSCPs).
Note : Advanced QoS methods only affect the internal priority queue mapping for the
Switch. The Switch does not modify the IEEE 802.1p value for the egress frames. You can
choose one of these ways to alter the way incoming packets are prioritized or you can
choose not to use any QoS enhancement setting on the Switch.
802.1p Priority
When using 802.1p priority mechanism, the packet is examined for the presence of a valid
802.1p priority tag. If the tag is present, the packet is assigned to a programmable egress
queue based on the value of the tagged priority. The tagged priority can be designated to
any of the available queues.
Ethernet Packet:
6 6 2 42-1496 4
DA SA Type / Length Data FCS
6 6 4 2 42-1496 4
DA SA 802.1Q Tag Type / Length Data FCS
802.1Q Tag:
2 bytes 2 bytes
Tag Protocol Identifier (TPID) Tag Control Information (TCI)
16 bits 3 bits 1 bit 12 bits
TPID (0x8100) Priority CFI VID

Tag Protocol Identifier (TPID): a 16-bit field set to a value of 0x8100 in order to
identify the frame as an IEEE 802.1Q-tagged frame.

Tag Control Information (TCI)

Priority Code Point (PCP): a 3-bit field which refers to the IEEE 802.1p priority.
It indicates the frame priority level from 0 (lowest) to 7 (highest) , which can be
used to prioritize different classes of traffic (voice, video, data, etc.).

Canonical Format Indicator (CFI): a 1-bit field. If the value of this field is 1, the
MAC address is in non-canonical format. If the value is 0, the MAC address is
in canonical format. It is always set to zero for Ethernet switches. CFI is used for
compatibility between Ethernet and Token Ring networks. If a frame received at
an Ethernet port has a CFI set to 1, then that frame should not be bridged to an
untagged port.

VLAN Identifier (VID): a 12-bit field specifying the VLAN to which the frame
belongs. A value of 0 means that the frame doesn't belong to any VLAN; in this
case the 802.1Q tag specifies only a priority and is referred to as a priority tag.
A value of hex 0xFFF is reserved for implementation use. All other values may