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I have a very basic background on networking Cisco (2960 switch), and realize that Fortiswitch 224E has the option to allow many VLANs in a port:

enter image description here On the other hand, is the trunk (which I assume is the same as Cisco):

enter image description here

I wonder what are the differences between them. Why a port would allow many VLANs?

Thanks in advance.

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Basically, a port with all VLANs allowed is a VLAN trunk. A trunk doesn't need to allow all VLANs, however.

Trunk used to be the port mode for multiple (tagged) VLANs, Cisco jargon. Other vendors use different terms for the same thing.

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A trunk (in common usage) carries frames with 802.1q tags to identify VLAN membership. In your example above, a given frame might have any of the allowed VLAN numbers as tags and the switch will forward these frames based on the forwarding tables maintained for their respective VLAN's.

A non-trunk port carries frames without an 802.1q tag. All frames received on such ports are associated to a single VLAN, and thus a single forwarding table. End hosts (PC's, for example) typically generate untagged traffic and switches will strip tags off of frames before transmitting them via a port to said end host.

To forestall another question - the idea of native VLAN is basically how untagged traffic received on a trunk port should be handled. In the case above, untagged traffic would be treated as if it were in VLAN 199 and, in turn, any hosts observed sending untagged traffic on this trunk port would receive untagged traffic as if they were in VLAN 199.

The opposite case (tagged traffic showing up on an access port) is treated as an error and generally discarded on modern switches after some notification/alarm is raised (indeed, in some cases the port is proactively shut down).

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    This is a good answer however what is missing, as Ron noted in a comment below: different vendors reuse words for different things. Jorge_S7 said "the trunk (which I assume is the same as Cisco)" - it's not. What a Fortinet's FortiSwitch calls a Trunk is what Cisco would call Port-Channel / Ether-Channel. So a "Trunk" in this case (which is a link aggregation) can have multiple VLANs and/or a native VLAN, same as any other (non-aggregated) port. I don't know if there's a specific FortiSwitch name for a port/trunk that has multiple VLANs vs one (the way Cisco has Trunk, Access, Hybrid, etc). – PSaul Oct 2 at 14:10
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There are two port types: Access and Trunk. Access carries data only for one VLAN but Trunk can carry data for multiple (or even all) VLANs.

  • Remember that "trunk" is a Cisco term. Other manufacturers use the term differently. For example, "Trunk"on HP is what Cisco calls a "port-channel." – Ron Trunk Oct 2 at 12:11
  • Port Channel is different concept and it is used in link aggregations – Mahdi Oct 7 at 7:44

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