42
Preamble
SFD
DA
SA
Type/LEN
PAYLOAD
FCS
Original frame
Preamble
SFD
DA
SA
TAG
TCI/P/C/VID
Type/LEN
PAYLOAD FCS
802.1q
frame
PRE Preamble
62 bits
Used to synchronize traffic
SFD Start Frame Delimiter
2 bits
Marks the beginning of the header
DA Destination Address
6 bytes
The MAC address of the destination
SA Source Address
6 bytes
The MAC address of the source
TCI Tag Control Info
2 bytes set to 8100 for 802.1p and Q tags
P Priority
3 bits
Indicates 802.1p priority level 0-7
C Canonical Indicator
1 bit
Indicates if the MAC addresses are in
Canonical format - Ethernet set to "0"
VID VLAN Identifier
12 bits
Indicates the VLAN (0-4095)
T/L Type/Length Field
2 bytes
Ethernet II "type" or 802.3 "length"
Payload < or = 1500 bytes User data
FCS Frame Check Sequence
4 bytes
Cyclical Redundancy Check
Important
VLAN Concepts for 802.1Q VLAN Configuration:
There are two key concepts to understand.
-
Access-VLAN
specifies the VLAN ID to the switch port that will assign the VLAN ID to
untagged
traffic from that port. A port can only be assigned to one Access-VLAN at a time. When
the port is configured as
Access Mode
, the port is called an
Access Port
, the link to/from this port
is called an
Access Link
. The VLAN ID assigned is called
PVID
.
-
Trunk-VLAN
specifies the set of VLAN IDs that a given port is allowed to receive and send
tagged
packets. A port can be assigned to multiple Trunk-VLANs at a time. When the port is
configured as
Trunk Mode
, the port is called a
Trunk Port
, the link to/from this port is called a
Trunk Link
. The VLAN ID assigned is called
VID
.
A port can be configured as below 802.1q VLAN modes :
-
Access Mode :
Access Links (the link to/from access ports) are the most common type of links on any VLAN
switch. All
network hosts (such as PCs)
connect to the switch's Access Links in order to gain
access to the local network. We configure only one
Access-VLAN
per port, that is, the VLAN ID
the
network hosts
will be allowed to access.
It is important to note at this point that any
network host
connected to an Access Port is totally
unaware of the VLAN assigned to the port. The
network host
simply assumes it is part of a single
