Modular Ethernet Switch User’s Guide
Switch Management Concepts
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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 straight-forward 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 2048 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).
