ZyXEL Communications XS1920 Series User Manual page 87

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 four bytes longer than an untagged frame and contains two
bytes of TPID (Tag Protocol Identifier, residing within the type/length field of the Ethernet frame)
and two 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 4,096 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 4,094.
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.
9.1.2.1 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.
Chapter 9 VLAN
CFI VLAN ID
1 Bit 12 bits
XS1920 Series User's Guide
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