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Chapter 57 VLAN
XGS2220 Series User’s Guide
416
57.2 Introduction to IEEE 802.1Q Tagged VLANs
A tagged VLAN uses an explicit tag (VLAN ID) in the MAC header to identify the VLAN membership of a
frame across bridges
– they are not confined to the switch on which they were created. The VLANs can
be created statically by hand or dynamically through GVRP. The VLAN ID associates a frame with a
specific VLAN and provides the information that switches need to process the frame across the network.
A tagged frame is 4 bytes longer than an untagged frame and contains 2 bytes of TPID (Tag Protocol
Identifier, residing within the type or length field of the Ethernet frame) and 2 bytes of TCI (Tag Control
Information, starts after the source address field of the Ethernet frame).
The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet switches. 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. The remaining twelve bits define the VLAN ID, giving a possible maximum number of
4096 VLANs. Note that user priority and VLAN ID are independent of each other. A frame with VID (VLAN
Identifier) of null (0) is called a priority frame, meaning that only the priority level is significant and the
default VID of the ingress port is given as the VID of the frame. 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 4094.
Forwarding Tagged and Untagged Frames
Each port on the Switch is capable of passing tagged or untagged frames. To forward a frame from an
802.1Q VLAN-aware switch to an 802.1Q VLAN-unaware switch, the Switch first decides where to
forward the frame and then strips off the VLAN tag. To forward a frame from an 802.1Q VLAN-unaware
switch to an 802.1Q VLAN-aware switch, the Switch first decides where to forward the frame, and then
inserts a VLAN tag reflecting the ingress port's default VID. The default PVID is VLAN 1 for all ports, but this
can be changed.
A broadcast frame (or a multicast frame for a multicast group that is known by the system) is duplicated
only on ports that are members of the VID (except the ingress port itself), thus confining the broadcast to
a specific domain.
57.2.0.1 Automatic VLAN Registration
GARP and GVRP are the protocols used to automatically register VLAN membership across switches.
GARP
GARP (Generic Attribute Registration Protocol) allows network switches to register and de-register
attribute values with other GARP participants within a bridged LAN. GARP is a protocol that provides a
generic mechanism for protocols that serve a more specific application, for example, GVRP.
GARP Timers
Switches join VLANs by making a declaration. A declaration is made by issuing a Join message using
GARP. Declarations are withdrawn by issuing a Leave message. A Leave All message terminates all
registrations. GARP timers set declaration timeout values.
TPID
16 Bits
User Priority
3 Bits
CFI
1 Bit
VLAN ID
12 Bits
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