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24-port NWay Ethernet Switch User’s Guide
22 Switch Management Concepts
the utmost flexibility. MAC-based broadcast domains, on the other hand, allow a station to be physically moved yet still
belong to the same broadcast domain without having to change and configuration settings.
The untagging feature of IEEE 802.1Q VLANs allows VLANs to work with legacy switches that don’t recognize VLAN
tags in packet headers. The tagging feature allows VLANs to span multiple 802.1Q-compliant switches through a single
physical connection and allows Spanning Tree to be enabled on all ports and work normally (BPDU packets are not
tagged).
MAC-based Broadcast Domains
The Switch supports up to 12 MAC-based broadcast domains, which are by their nature, limited to the Switch itself and
the devices connected directly to it.
Since MAC addresses are hard-wired into a station’s network interface card (NIC), MAC-based broadcast domains enable
network managers to move a station to a different physical location on the network and have that station automatically
retain its broadcast domain membership. This provides the network with a high degree of flexibility since even notebook
PC’s can plug into any available port on a network and communicate with the same people and use the same resources
that have been allocated to the broadcast domain in which it is a member.
Since MAC-based broadcast domains do not restrict the transmission of known unicast frames to other broadcast
domains, they can only be used to define limited broadcast domains. As such, they are best implemented on networks
where stations are frequently moving, for example where people using notebook PCs are constantly plugging into different
parts of the network.
Setting up MAC-based broadcast domains is a relatively straightforward process. Simply create the broadcast domain by
assigning it a name (description) and add MAC addresses for the stations that will be members.
802.1Q VLANs
The Switch supports up to 2000 802.1Q VLANs. 802.1Q VLANs limit traffic that flows into and out of switch ports. Thus,
all devices connected to a port are members of the VLAN(s) the port belongs to, whether there is a single computer directly
connected to a switch, or an entire department.
On 802.1Q VLANs, NICs do not need to be able to identify 802.1Q tags in packet headers. NICs send and receive normal
Ethernet packets. If the packet’s destination lies on the same segment, communications take place using normal Ethernet
protocols. Even though this is always the case, when the destination for a packet lies on another Switch port, VLAN
considerations come into play to decide if the packet gets dropped by the Switch or delivered.
There are two key components to understanding 802.1Q VLANs: Port VLAN ID numbers (PVIDs) and VLAN ID numbers
(VIDs). Both variables are assigned to a switch port, but there are important differences between them. A user can only
assign one PVID to each switch port. The PVID defines which VLAN a packet belongs to when packets need to be
forwarded to another switch port or somewhere else on the network. On the other hand, a user can define a port as a
member of multiple VLANs (VIDs), allowing the segment connected to it to receive packets from many VLANs on the
network. These two variables control a port’s ability to transmit and receive VLAN traffic, and the difference between
them provides network segmentation, while still allowing resources to be shared across more than one VLAN.
802.1Q VLAN Segmentation
The following example is helpful in explaining how 802.1Q VLAN segmentation works. Take a packet that is transmitted
by a machine on Port 1 that is a member of VLAN 2 and has the Port VLAN ID number 2 (PVID=2). If the destination lies
on another port (found through a normal forwarding table lookup), the Switch then looks to see if the other port (Port 10) is
a member of VLAN 2 (and can therefore receive VLAN 2 packets). If port 10 is not a member of VLAN 2, then the packet
will be dropped by the Switch and will not reach its destination. If Port 10 is a member of VLAN 2, the packet will go
through. This selective forwarding feature based on VLAN criteria is how VLANs segment networks. The key point being
that Port 1 will only transmit on VLAN 2, because it’s Port VLAN ID number is 2 (PVID=2).