Specifications

Switching 8-15
Software Release 2.7.3
C613-03098-00 REV A
state. This effectively disables the guest VLAN on the port until the port’s link
goes down.
A guest VLAN can only be configured for a port that is running in single-
supplicant mode.
VLAN Tagging
An Ethernet packet can contain a VLAN tag with fields that specify VLAN
membership and user priority. The VLAN tag is described in IEEE
Standard 802.3ac, and is four octets that can be inserted between the Source
Address and the Type/Length fields in the Ethernet packet (Figure 8-1 on
page 8-16). To accommodate the tag, IEEE Standard 802.3ac also increased the
maximum allowable length for an Ethernet frame to 1522 octets (the minimum
size is 64 octets). IEEE Standard 802.1q specifies how the data in the VLAN tag
switches frames. VLAN-aware devices are able to add the VLAN tag to the
packet header. VLAN-unaware devices cannot set or read the VLAN tag.
Table 8-2 on page 8-15 lists the meaning and use of the fields in the Ethernet
frame. Figure 8-1 on page 8-16 shows the format of VLAN data in an Ethernet
frame. Twelve bits of the tag are the VLAN Identifier (VID), which indicates the
VLAN to which the packet belongs. Table 8-3 on page 8-16 lists the VLAN
Identifier values that have specific meaning.
Table 8-2: Fields in the Ethernet frame for QoS and VLAN switching
Field Length Meaning and use
TPID 2 octets The Tag Protocol Identifier (TPID) is defined by IEEE
Standard 802.1q as 0x81-00.
User Priority 3 bits The User Priority field is the priority tag for the frame, which
can be used by the switch to determine the Quality of
Service to apply to the frame. The three bit binary number
represents eight priority levels, 0 to 7.
CFI 1 bit The Canonical Format Indicator (CFI flag) indicates whether
all MAC address information that may be present in the
MAC data carried by the frame is in canonical format.
VID 12 bits The VLAN Identifier (VID) field uniquely identifies the VLAN
to which the frame belongs.