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We know that a switch can be divided logically into different lans by using VLAN using VLAN tagged mechanism. Now is it necessary that in each vlan to tagg a port? Also what is the use of tagged port?

My question is different in the sense as the other solution tells what is vlan tagging but it doesnot clearly mention how does the tagged port works or why do we nned to tag a port

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It is not necessary to have at least one port tagged per vlan. You can let all your ports untagged.

A tagged port is usually used when you have more than one vlan in it. To determine which vlan a frame is belonging to the switch is setting a tag. The tag is located in the ethernet header: https://en.wikipedia.org/wiki/IEEE_802.1Q

Typically switch uplinks are tagged, because they are configured with several vlans.

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We know that a switch can be divided logically into different lans by using VLAN using VLAN tagged mechanism.

VLAN tagging is not required to logically divide a switch into different LANs. VLAN tagging is only necessary when sending data from more than one VLAN on the switch to another switch or device over a single link.

Now is it necessary that in each vlan to tagg a port?

No, as I said above, it is only necessary when you are sending data from more than one VLAN across a single link.

Also what is the use of tagged port?

By now, you can probably guess where this answer is heading. The use of a tagged port (aka trunk port for some vendors) is to allow you to send the data from more than one VLAN over a single link. The tagging allows the receiving device to understand which traffic belongs to which VLAN allowing it to maintain the logical separation.

For normal access ports, there should only be one (untagged) VLAN configured and no addition (tagged) VLANs.

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The switch, by configuration knows which port belongs to which VLAN, but when a frame coming from a PC have to go to another switch it has to use the common link between them.

There is not other way than "tagging" the frame before sending it on the common link between switches and stripping the tag before sending it out to the destination PC.

Using 802.1Q a tag containing the VLAN-ID can be put inside the ethernet frame before the frame goes to the common link.

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  • do you mean that the common link is the tagged port? – Kalu Nov 20 '15 at 11:19
  • Yes. Regular ethernet ports on PCs, servers,etc. can't understand VLAN tags. Ports connecting those devices are untagged. Ports that are used for switch interconnection must be tagged (also called VLAN Trunking) to be able to transport multiple VLAN traffic from switch to switch. Also, you can have a tagged port from switch to a router (the router must support VLAN Trunking) – jcbermu Nov 20 '15 at 11:28
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Is it necessary to tag a port? No, not at all. A switch works right out of the box with no VLAN tagging necessary.

"What's the purpose of VLAN tagging?"

Separate broadcast domains within a Layer 2 network. In simple terms, separate the Switch logically into 2 or more separate networks. Benefits of VLANs include, increased security and increased bandwidth.

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