Specifications

34
TCI (Tag Control Information field) including user priority, Canonical format indicator (CFI) and
VLAN ID.
TPID- defined value of 8100 in hex. When a frame has the EtherType equal to 8100, this frame
carries the tag IEEE 802.1Q / 802.1P.
User Priority- Defines user priority, giving eight (2
3
= 8) priority levels. IEEE 802.1P defines the
operation for these 3 user priority bits.(Refer to following table)
CFI (Canonical Format Indicator) is always set to zero for Ethernet switches. CFI is used for
compatibility reasons between Ethernet type network and Token Ring type networks. If a
frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded
as it is to an untagged port.
VID- VLAN ID is the identification of the VLAN, which is basically used by the standard 802.1Q.
It has 12 bits and allow the identification of 4096 (2
12
) VLANs. Of the 4096 possible VIDs, a VID
of 0 is used to identify priority frames and value 4095 (FFF) is reserved, so the maximum
possible VLAN configurations are 4,094.
The EFM modem by default initially configures one VLAN, VID=1.
A port such as LAN1 to 4, DSL or sniffing can have only one PVID, but can have as many VID
as the EFM modem has memory in its VLAN table to store them.
Ports in the same VLAN group share the same frame broadcast domain and thus increase
network performance through reduced boardcast traffic. VLAN groups can be modified at any
time by adding, moving or changing ports without any re-cabling.