ZyXEL Communications XS1930-12F User Manual page 120

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.
TPID User Priority
2 Bytes 3 Bits
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.
9.2.0.1 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 Declarations are withdrawn by issuing a Leave message.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.
GVRP
GVRP (GARP VLAN Registration Protocol) is a registration protocol that defines a way for switches to
register necessary VLAN members on ports across the network. Enable this function to permit VLAN
groups beyond the local Switch.
Chapter 9 VLAN
CFI VLAN ID
1 Bit 12 bits
XS1930 Series User's Guide
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