Forwarding Tagged And Untagged Frames; Automatic Vlan Registration; Garp - ZyXEL Communications AAM1212 User Manual

AAM1212 User's Guide
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 AAM handles up to 4094 VLANs (VIDs 1-4094). The switch accepts incoming frames
with VIDs 1-4094.

9.2.1 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. For a
broadcast frame, it is duplicated only on ports (except the ingress port itself) that are members
of the VID, 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.

9.3 Automatic VLAN Registration

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

9.3.1 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.
112
CFI VLAN ID
1 Bit 12 bits
Chapter 9 VLAN