4.1 Understanding Trunk and Access Ports
VLANs can be assigned to trunk ports and access ports on a network. These two types of allocation
determine how data is transmitted and relayed.
4.1.1 Trunk Ports
Trunk ports typically provide a connection between network switches, and can carry data for multiple
VLANs. They will only transmit frames (packets of data) that belong to the port's assigned VLANs. To
identify the VLAN of each frame, a network switch adds a tag to the frame (known as 802.1Q trunking).
The tag contains the following information:
VLAN ID — allows the network switch receiving a frame to identify the VLAN it belongs to.
Priority ID — allows the network switch to prioritise distribution when multiple frames are being
transmitted. Priority ID ranges from 0-7, where 7 is the highest priority.
When a network switch receives a tagged frame, the tag is read to determine the VLAN it belongs to. The
tag is removed and distributed to devices connected on the same VLAN.
When the network switch receives multiple frames, it will prioritise the distribution of frames based on the
Priority ID in the VLAN ID tag. For more information on configuring VLANs, see Defining VLANs on page
77.
4.1.2 Access Ports
Access ports connect client devices such as PCs and laptops to the network switch, and can only be
assigned to a single VLAN. Access ports can only send and receive untagged frames, with those frames
allocated to the relevant VLAN inside the switch. Any tagged frames sent to an access port will be dropped.
An example of VLAN traffic flow through trunk and access ports is shown in Figure 15: VLAN traffic flow
and described below.
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