Hey SushilI will do my best. Whether a port is tagged or not is dependent mainly on how many VLANs are on a particular port. The standard for this is based on 802.1Q.The standard states that on any given port you can have one untagged VLAN. This means that you can have one VLAN per port and there is no need to tag the port. Tagging means that the port will send out a packet with a header that has a tag number that matches the VLAN tag number. If there is only one VLAN there is no need to distinguish what VLAN the traffic is for as there is only one. When there are more than one VLAN per port the port must differentiate what VLAN each packet is destined to. So for example if we have 3 VLANs on port 1 VL1, VL2, VL3 and I am sending out a packet on each one I have to have a way to tell the other side which packet is for which VLAN. So if the .1Q tag for VL1 is 1 and for VL2 is 2 and VL3 is 3 then the port will insert a header with the right tag number on each packet. The other side needs to be set up the same way and will see that if a packet of 1 comes in it will forward it on VL1.So when a port is tagged it inserts and receives packets with the 802.1Q tag for every packet that has a VLAN on it.The standard does allow for one untagged VLAN per port as mentioned above which means that you could have VL1 with no tag and VL2 and VL3 with tags because technically all three VLANs are unique. any packet that has no tag will go to VL1. This is what Cisco does with their native VLAN.Hope this helps.P (from Paul_Russo)
