Allied Telesis AT-S101 User Manual
Page 49
AT-S101 Management Software User’s Guide
49
Tagged and Untagged Ports
When you specify that a port is a member of a tagged VLAN, you need to
specify that it is tagged or untagged. You can have a combination of
tagged and untagged ports in the same VLAN.
Packet transmission from a tagged port differs from packet transmission
from an untagged port. When a packet is transmitted from a tagged port,
the tagged information within the packet is maintained when it is
transmitted to the next network device. If the packet is transmitted from an
untagged port, the VLAN tag information is removed from the packet
before it is transmitted to the next network device.
The IEEE 802.1Q standard describes how tagging information within a
packet is used to forward or discard traffic throughout the switch. If the
incoming packet has a VLAN tag that matches one of the Group IDs of
which the port is a member, the packet is accepted and forwarded to the
appropriate port(s) within that VLAN. If the incoming packet’s VLAN tag
does not match one of the Group IDs assigned to the port, the packet is
discarded.
Port VLAN Identifier
When an untagged packet is received on a port in a tagged VLAN, it is
assigned to one of the VLANs of which that port is a member. The
deciding factor in this process is the Port VLAN Identifier (PVID). Both
tagged and untagged ports in a tagged VLAN must have a PVID assigned
to them. The default value of the PVID for each port is 1. The switch
associates a received untagged packet to the Group ID that matches the
PVID assigned to the port. As a result, the packet is only forwarded to
those ports that are members of that VLAN.
General Rules for Creating a Tagged VLAN
Below is a summary of the rules to observe when you create a tagged
VLAN.
Each tagged VLAN must be assigned a unique VID. If a particular
VLAN spans multiple switches, each part of the VLAN on the different
switches must be assigned the same VID.
A tagged port can be a member of multiple VLANs.
The AT-GS950/8POE switch can support up to 200 tagged VLANS.