IEEE 802.1Q: Difference between revisions

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IEEE 802.1Q was a project in the IEEE 802 standards process to develop a mechanism to allow multiple bridged networks to transparently share the same physical network link without leakage of information between networks (i.e. "trunking"). IEEE 802.1Q is also the name of the standard issued by this process, and in common usage the name of the encapsulation protocol used to implement this mechanism over Ethernet networks.

IEEE 802.1Q also defines the meaning of a virtual LAN or VLAN with respect to the specific conceptual model underpinning bridging at the MAC layer and to the IEEE 802.1D spanning tree protocol. This protocol allows for individual VLANS to communicate with one another with the use of a layer-3 (network) router. See also Cisco's proprietary VTP.

Frame format

802.1Q does not actually encapsulate the original frame. Instead, it adds an extra 4-byte header to the original Ethernet header. The EtherType is changed to 0x8100, denoting the new frame format. The header contains the following fields:

user_priority: this 3-bit field can be used to store a priority level for the frame. Use of this field is defined in IEEE 802.1p.
CFI: a 1-bit flag denoting whether MAC addresses in the frame are in canonical format. This is called the Canonical Format Indicator.
VID: a 12-bit VLAN ID, allowing up to 4096 VLANs.

Because inserting this header changes the frame, 802.1Q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer.

Native VLAN

Clause 9 of the standard defines the encapsulation protocol used to multiplex VLANs over a single link, and introduces the concept of a native VLAN. Frames belonging to the native VLAN are not modified when sent over the trunk. Native VLANs are also referred to as "Management VLANs".

Eg. Say you had a .1q port with VLANs 2, 3 and 4 asigned to it. If Vlan 2 is the Native VLAN, frames on this VLAN which are to egress this port are not given a .1q header ie. they are plain ethernet frames. Frames which ingress this port which have no .1q header are put into VLAN 2. Behaviour of traffic relating to VLANs 3 & 4 is intuitive.

You can (obviously) only have one Native VLAN per port

Generic Attribute Registration Protocol

In addition, IEEE 802.1Q defines GVRP, an application of the Generic Attribute Registration Protocol, allowing bridges to negotiate the set of VLANs to be trunked over a specific link.

Multiple spanning-tree protocol

The 2003 revision of the standard also rolled in the Multiple Spanning Tree Protocol (MSTP) originally defined in IEEE 802.1s.

The official title of the 802.1Q standard is IEEE Std. 802.1Q-2003, Virtual Bridged Local Area Networks; ISBN 0-7381-3662-X.

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External links

802.1Q-2003 standard (must accept license agreement)

@@ Line 1: / Line 1: @@
 '''IEEE 802.1Q''' was a project in the [[IEEE 802]] standards process to develop a mechanism to allow multiple [[network bridge|bridge]]d networks to transparently share the same physical network link without leakage of information between networks (i.e. "trunking").  IEEE 802.1Q is also the name of the standard issued by this process, and in common usage the name of the encapsulation protocol used to implement this mechanism over [[Ethernet]] networks.
-IEEE 802.1Q also defines the meaning of a  '''virtual [[LAN]]''' or [[VLAN]] with respect to the specific conceptual model underpinning bridging at the [[Media Access Control|MAC]] layer and to the [[IEEE 802.1D]] [[spanning tree (networks)|spanning tree]] protocol.
+IEEE 802.1Q also defines the meaning of a  '''virtual [[LAN]]''' or [[VLAN]] with respect to the specific conceptual model underpinning bridging at the [[Media Access Control|MAC]] layer and to the [[IEEE 802.1D]] [[spanning tree (networks)|spanning tree]] protocol.  This protocol allows for individual VLANS to communicate with one another with the use of a layer-3 (network) router.  See also Cisco's proprietary [[VTP]].
 == Frame format ==
@@ Line 15: / Line 15: @@
 == Native VLAN ==
-Clause 9 of the standard defines the encapsulation protocol used to [[multiplexing|multiplex]] VLANs over a single link, and introduces the concept of a native VLAN. Frames belonging to the native VLAN are not modified when sent over the trunk.
+Clause 9 of the standard defines the encapsulation protocol used to [[multiplexing|multiplex]] VLANs over a single link, and introduces the concept of a native VLAN. Frames belonging to the native VLAN are not modified when sent over the trunk.  Native VLANs are also referred to as "Management VLANs".
 Eg.