Forwarding Tagged And Untagged Frames; Automatic Vlan Registration; Garp - ZyXEL Communications GS2200-24 User Manual

Chapter 9 VLAN
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

9.3.2 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.

9.3.3 Automatic VLAN Registration

GARP and GVRP are the protocols used to automatically register VLAN
membership across switches.

9.3.4 GARP

GARP (Generic Attribute Registration Protocol) allows network switches to register
and de-register attribute values with other GARP participants within a bridged
88
CFI VLAN ID
1 Bit 12 bits
GS2200-24 User's Guide