Forwarding Tagged And Untagged Frames; Garp Timers; Automatic Vlan Registration; Garp - ZyXEL Communications ALC1024 User Manual

Web Configurator with EEC 2
19.3

Forwarding Tagged and Untagged Frames

Each port on the switch is capable of receiving tagged or untagged frames. You can configure a card to receive only
tagged or all frames on a port-by-port basis. If it is set to tagged-only on a port, then only tagged frames are allowed
to enter from that port and untagged frames are dropped; if set to all, then both tagged and untagged frames are
allowed to enter the switch. The card does not alter the VID of a frame if it is already tagged; however, when an
untagged frame enters the switch, it is assigned the default port VID (PVID) of the ingress (incoming) port. Thus a
frame always has a VID inside the switch, regardless of whether it is tagged or not on the wire. The default PVID is
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 (or multicast without IGMP snooping) 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 a per VLAN, per port basis (recall
that a port can be members of multiple VID). 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.
19.4

Automatic VLAN Registration

GARP and GVRP are the protocols used to automatically register VLAN membership across switches.
19.5

GARP

GARP (Generic Attribute Registration Protocol) allows network devices 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.
19.5.1

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Declarations are withdrawn by issuing a Leave message.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.
19.5.2 GVRP
GVRP (GARP VLAN Registration Protocol) is a registration protocol that defines a way for switches to register
necessary VLAN members on ports across the network. Enable this function to permit VLANs groups beyond the
local switch.
Please refer to the following table for common GARP terminology.
19-2 VLAN