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

MSC1000G User's Guide
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
The MSC 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.
250
CFI VLAN ID
1 Bit 12 bits
Chapter 15 VLAN Screens