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SME VOIP SYSTEM GUIDE 1.1 116 | P a g e
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12.2 Backbone/ VLAN Aware Switches
To implement a VLAN in your network, you must use VLAN-aware switches.
Before we continue, let consider two rules to remember regarding the functioning of a regular LAN switch:
1. When the switch receives a broadcast or multicast frame from a port, it floods (or broadcasts) the frame to
all other ports on the switch.
2. When the switch receives a unicast frame, it forwards it only to the port to which it is addressed.
A VLAN-aware switch changes the above two rules as follows:
1. When the switch receives a broadcast or multicast frame from a port, it floods the frame to only those ports
that belong to the same VLAN as the frame.
2. When a switch receives a unicast frame, it forwards it to the port to which it is addressed, only if the port
belongs to the same VLAN as the frame.
3. A unique number called the VLAN ID identifies each VLAN.
Which VLAN Does a Frame Belong To?
The previous section notes that a frame can belong to a VLAN. The next question is—how is this association made?
A VLAN-aware switch can make the association based on various attributes of the type of frame, destination
of MAC address, IP address, TCP port, Network Layer protocol, and so on.
An illustration of IEEE 802.1Q VLAN tag in Ethernet frame is as follows:
12.3 How VLAN Switch Work: VLAN Tagging
VLAN functionality can be implemented via explicit frame tagging by switches and end stations. Network switches and
end stations that know about VLANs are said to be VLAN aware. Network switches and end stations that can interpret
VLAN tags are said to be VLAN tag aware. VLAN-tag-aware switches and end stations add VLAN tags to standard
Ethernet frames–a process called explicit tagging. In explicit tagging, the end station or switch determines the VLAN
membership of a frame and inserts a VLAN tag in the frame header (see figure above for VLAN tagging), so that
downstream link partners can examine just the tag to determine the VLAN membership.