AT-S41 User’s Guide
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Port-based VLAN Mode Overview
The AT-8326GB switch features a special Port-based VLAN mode. This
VLAN mode allows you to create VLANs that are slightly different than
the tagged and untagged VLANs described earlier in this chapter.
Note
For those of you who are familiar with Allied Telesyn products,
please note that the port-based VLAN described here is not the
same as the port-based VLANs featured in our other managed
switches, such as the AT-8024 Fast Ethernet switch. The untagged
VLAN described earlier in this chapter is analogous to the port-
based VLAN featured in other Allied Telesyn switch products.
Port-based VLANs are just lists of ports that belong to different VLANs on
the switch. To create a port-based VLAN, you simply indicate which ports
you want in it. You do not configure PVIDs, as you do for untagged ports,
and, while you do specify a unique VID when you create a port-based
VLAN, its use is limited to within the switch. A VID is not used across
multiple switches to identify different port-based VLANs.
Port-based VLANs do have a couple of advantages over tagged and
untagged VLANs described earlier in this chapter. For instance, port-
based VLANs are easier to configure, mainly because you do not have to
worry about setting PVIDs.
Plus, it can be easier to share network resources. Ports can be shared in
port-based VLANs and the shared devices do not need to be IEEE
802.1Q-compliant, as required with a tagged VLAN.
The major disadvantage to port-base VLANs is VLAN leakage, where
frames that originate in one VLAN end up in another. This can occur
where port-based VLANs share ports.
Here is how this can occur. Let’s assume that a frame arrives on a switch
port that is shared among three different VLANs. If the destination MAC
address in the frame has not been learned by the switch, the frame will
be broadcast out the ports of all three VLANs. Hence, the frame crosses
the boundaries of the VLANs.
This might not be a problem for you if network security is not a major
concern. However, if it is a high priority, then either the VLANs that you
create with the port-based VLAN mode should not contain shared ports,
or you should instead create tagged VLANs, as described earlier in this
chapter.
