Canonical format - Ethernet set to "0"
Indicates the VLAN (0-4095)
Ethernet II "type" or 802.3 "length"
Payload < or = 1500 bytes User data
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
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 broadcast domain, just as it happens with any normal switch. During data transfers,
any VLAN information or data from other VLANs is removed so the recipient has no
information about them.
- Trunk Mode :
Trunk Links (the link to/from trunk ports) is configured to carry packets for multiple VLANs.
These types of ports are usually found in connections between switches. These links require
the ability to carry packets from multiple VLANs because VLANs span over multiple switches.
- Trunk Native Mode :
A Trunk-native port can carry untagged packets simultaneously with the 802.1Q tagged
packets. When you assign a default Access-VLAN to the trunk-native port, all untagged traffic
travels on the default Access-VLAN for the trunk-native port, and all untagged traffic is
assumed to belong to this Access-VLAN. This Access-VLAN is referred to as the native VLAN
ID for a Trunk-native Port. The native VLAN ID is the VLAN ID that carries untagged traffic on
trunk-native ports.
- DOT1Q-Tunnel Mode :
Business customers of service providers often have specific requirements for VLAN IDs and
the number of VLANs to be supported. The VLAN ranges required by different customers in
the same service-provider network might overlap, and traffic of customers through the
infrastructure might be mixed. Assigning a unique range of VLAN IDs to each customer would
restrict customer configurations and could easily exceed the VLAN limit (4096) of the IEEE
802.1Q specification.
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