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A trunk port is a specific type of port on a network switch that allows data to flow across a network node for multiple virtual local area networks or VLANs. Think of the trunk port as a “bundle” of individual branches or capillaries in a telecom network connection.
The typical VLAN network is made up of virtualized network nodes. By contrast, the traditional network was a series of pieces of hardware connected together, where each one was its own network node.
Now, by the principle of virtualization, these pieces of hardware can be endowed with virtual partitions through “extra logic,” to change how they handle data.
The “trunk port” relies on the basic idea of “trunking” — that a single point-to-point connection can handle two different kinds of traffic. In this case, that means handling traffic for more than one VLAN.
Trunking as a concept was popularized in PBX and VoIP with SIP trunking and other telecom innovations to allow a single gateway to deliver data and voice to multiple individual user endpoints. Now it's being used to facilitate multiple VLAN traffic over virtualized network nodes.
The trunking of the involved VLANs takes place in the layer 2 of the OSI model, which designates seven different layers for network transmissions. Layer 2 is known as the "data link layer" and is typically used for MAC broadcasting and signaling between nodes of an internal network.
By contrast, layer 3 is often used for segmented IP routing. The basic way to look at this is that layer 2 is much more geared toward internal network activities, where layer 3 utilizes the IP system in a more external way.
In order to accommodate VLAN trunking, the trunk port routes a data packet that contains a tag in the frame header. This tag designates which VLA the packet is supposed to be routed to.
This is the fundamental way that the system knows where to send each packet as it proceeds through the common trunk. These unique identifying tags are either 802.1Q tags or Inter-Switch Link (ISL) tags.
It’s interesting to note that the trunk port can carry untagged packets simultaneously with tagged packets.
It's important to note that in using trunk ports for VLAN trunking, the default is that all data flows to all of the VLANs. However, it's possible for engineers to manually calibrate the setups differently, where they can stop traffic from to and from an individual VLAN.
An Ethernet interface can either function as a trunk port or as an access port, but not both at the same time.
The versatility of a trunk port and a VLAN trunk can be shown through various data flow charts showing how the VLANs are chained together and use common resources.
In some ways, this hearkens back to the days of the hardware network topology, where individual pieces of hardware were connected in designated topologies or structures including a bus topology, a star topology, a ring topology or other discrete setups.