Forwarding Tagged And Untagged Frames; Automatic Vlan Registration - ZyXEL Communications MSC1000G User Manual

Chapter 15 VLAN Screens
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.
The system handles up to 4094 VLANs (VIDs 1-4094). The switch accepts incoming frames with
VIDs 1-4094.

15.2.1 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.
The egress (outgoing) port(s) of a frame is determined on the combination of the destination MAC
address and the VID of the frame. For a unicast frame, the egress port based by the destination
address must be a member of the VID, also; otherwise, the frame is blocked. 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.
Whether to tag an outgoing frame depends on the setting of the egress port on an individual VLAN
and port basis (remember that a port can belong to multiple VLANs). If the tagging on the egress
port is enabled for the VID of a frame, then the frame is transmitted as a tagged frame; otherwise,
it is transmitted as an untagged frame.

15.3 Automatic VLAN Registration

GARP and GVRP are the protocols used to automatically register VLAN membership across switches.
15.3.1 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.
15.3.1.1 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.
592
TPID User Priority
2 Bytes 3 Bits
CFI VLAN ID
1 Bit 12 bits
Management Switch Card User's Guide