Virtual LANs4-3
This switch supports the following VLAN features:
• Up to 256 VLANs can be configured based on the IEEE 802.1Q standard
• Distributed VLAN learning across multiple switches using explicit or implicit tagging
and GVRP protocol
• Port overlapping, allowing a port to participate in multiple VLANs
• End stations can belong to multiple VLANs
• Passing traffic between VLAN-aware and VLAN-unaware devices
• Priority tagging
• Port trunking with VLANs
Assigning Ports to VLANs
Before enabling VLANs for the switch, you must first assign each port to the VLAN
group(s) it will participate in. (By default all ports are assigned to VLAN 1 as
untagged ports.) Add a port as a tagged port (that is, a port attac hed to a
VLAN-aware device) if you want it to carry traffic for one or more VLANs and the
device at the other end of the link also supports VLANs. Then assign the port at the
other end of the link to the same VLAN(s). However, if you want a port on this switch
to participate in one or more VLANs, but the device at the other end of the l ink does
not support VLANs, then you must add this port as an untagged port (that is, a port
attached to a VLAN-unaware device).
Port-based VLANs are tied to specific ports. The switch’s forwarding decision is
based on the destination MAC address and its associated port. Therefore, to make
valid forwarding and flooding decisions, the switch learns the relationship of the
MAC address to its related port—and thus to the VLAN—at run-time. When the
switch receives a frame, it assigns the frame to the port’s default VLAN if the frame
is untagged (determined by the PVID of the receiving port), or maps it for output to
the broadcast domain associated with the frame’s VLAN tag.
Port Overlapping
Port overlapping can be used to allow access to commonly shared network
resources among different VLAN groups, such as file servers or printers. Note that if
you implement VLANs which do not overlap, but still need to communicate, you can
connect them using a router or Layer 3 switch.
Automatic VLAN Registration (GVRP)
GVRP defines a system whereby the switch can automatically learn the VLANs
each endstation should be assigned to. If an endstation (or its network adapter)
supports the IEEE 802.1Q VLAN protocol, it can be configured to broadcast a
message to your network indicating the VLAN groups it wants to join. When this
switch receives these messages, it will automatically place the receiving port in the
specified VLANs, and then forward the message to all other ports. When the
message arrives at another switch that supports GVRP, it will also place the
receiving port in the specified VLANs, and pass the message on to all other ports.
VLAN requirements are propagated in this way throughout the n etwork.