40
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 be used as VLAN identifiers,
allowing up to 4094 VLANs. On bridges, VLAN 1 is often reserved for management.
Priority Levels
PCP: Priority Code Point.
