TRENDnet TI-PG541i User Manual page 33

TRENDnet User's Guide
6 6 4
DA SA 802.1Q Tag
802.1Q Tag:
2 bytes
Tag Protocol Identifier (TPID)
16 bits
TPID (0x8100)

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.
PCP Network Priority
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2 42-1496
Type / Length Data
2 bytes
Tag Control Information (TCI)
3 bits 1 bit 12 bits
Priority CFI VID
Traffic Characteristics
1
4
0
FCS
2
3
4
5
6
7
DiffServ (DSCP)
Differentiated Services or DiffServ is a computer networking architecture that specifies a
simple, scalable and coarse-grained mechanism for classifying, managing network traffic
and providing Quality of Service (QoS) guarantees on modern IP networks. DiffServ can,
for example, be used to provide low-latency, guaranteed service (GS) to critical network
traffic such as voice or video while providing simple best-effort traffic guarantees to non-
critical services such as web traffic or file transfers.
Differentiated Services Code Point (DSCP) is a 6-bit field in the header of IP packets for
packet classification purposes. DSCP replaces the outdated IP precedence, a 3-bit field in
the Type of Service byte of the IP header originally used to classify and prioritize types of
traffic.
When using the DiffServ priority mechanism, the packet is classified based on the DSCP
field in the IP header. 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.
Version IHL
Identification
Time to Live
0 (lowest) Background
1 Best Effort
2 Excellent Effort
3 Critical Applications
4 Video, <100ms latency
5 Video, < 10ms latency
6 Internetwork Control
7 (highest) Network Control
Type of Service Total Length
Flags
Protocol Header Checksum
TI-PG541i
Fragment Offset
23