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Author Topic: difference between untagged VLAN and PVID  (Read 64309 times)

chrpinedo

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difference between untagged VLAN and PVID
« on: October 05, 2011, 12:33:34 AM »

Hello all,

this is my first post in the forum. We have more than 10 DGS-1210-24 switches in our LAN network and I don't understand well the difference between an untagged VLAN in a port and the PVID of a port.

I am more accustomed to Cisco switches. In Cisco switches there are only untagged and tagged VLAN in a port, not the PVID for a port.

So what is the difference between set a VLAN untagged in a port and set PVID for the same port???

Thanks,

Christian
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fles_dnoyeb

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Re: difference between untagged VLAN and PVID
« Reply #1 on: December 07, 2012, 03:41:00 AM »

Tagged/untagged has to do with how packets leave the switch.  PVID controls which VLAN untagged packets that enter the switch belong to.  In theory.

Sometimes with the behavior of the switch I am not so sure...
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Planck

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Re: difference between untagged VLAN and PVID
« Reply #2 on: January 02, 2013, 10:22:21 AM »

Here's the quick overview on this topic:

Tagged traffic: Is used between tag-aware units (Switches and AP's), so they can see what VLAN the traffic belongs to. In short, if you use one cable between two switches, you will have to tag traffic. You can have one untagged all the way around in the network, but the next VLAN's have to be tagged.

PVID: Port VLAN ID is used to untag traffic befor it leaves the switch. If you have one unit/PC, that you only wanna use in VLAN 4, then you set the port it is connected to to VLAN 4, and only that traffic will be send to that specific port.

Best practice:
VLAN 1 untagged in all of the network, and the rest is tagged. When you need to extract one VLAN to a specific port, you will need to assign the PVID to the correct VLAN.

If you have any further questions, then you can ask here: http://structureit.dk/forum/

Best regards
Planck
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dmx.tech

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Re: difference between untagged VLAN and PVID
« Reply #3 on: May 29, 2016, 06:51:36 AM »

This may help any body who is confused by these terms. Like i was and spent month reading and testing about it.

This can be helpful to you if you have 2 switch and want to use a single cable to link both switch  for many Vlan. If you have only 1 switch then  your network card need to support  802.11q. You can create multiple vlan in  your pc then connect to your switch. That link wil be called trunk.

I assume anybody who will read this already  know the basic of Vlan.  The port  that will connect to the PC or Other switch from your switch need to be set as  TAGGED. When you set it as tagged you instruct the switch that data packets that will go through it is already tagged. You can have many vlan data packets via a switch port with is set as tagged. Untagged ports are  member of  a vlan and PVID  is for non tagged packets arriving at a port on the switch.

If a packet arrives at the port from an end device carrying no VLAN tag, then the switch will add a VLAN tag which corresponds to the PVID, and then forward it within that VLAN; so the PVID mechanisim allows you to have traffic originating from a non-VLAN aware device to become an 802.1q packet, so that it can traverse to other switches and still be contained within the correct VLAN.
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xorken

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Re: difference between untagged VLAN and PVID
« Reply #4 on: November 27, 2016, 10:37:52 AM »

This may help any body who is confused by these terms. Like i was and spent month reading and testing about it.

This can be helpful to you if you have 2 switch and want to use a single cable to link both switch  for many Vlan. If you have only 1 switch then  your network card need to support  802.11q. You can create multiple vlan in  your pc then connect to your switch. That link wil be called trunk.

I assume anybody who will read this already  know the basic of Vlan.  The port  that will connect to the PC or Other switch from your switch need to be set as  TAGGED. When you set it as tagged you instruct the switch that data packets that will go through it is already tagged. You can have many vlan data packets via a switch port with is set as tagged. Untagged ports are  member of  a vlan and PVID  is for non tagged packets arriving at a port on the switch.

If a packet arrives at the port from an end device carrying no VLAN tag, then the switch will add a VLAN tag which corresponds to the PVID, and then forward it within that VLAN; so the PVID mechanisim allows you to have traffic originating from a non-VLAN aware device to become an 802.1q packet, so that it can traverse to other switches and still be contained within the correct VLAN.
Hello, im still not sure if I fully understand the concept of PVID.

Take a Cisco switch for example, a port that is set to be member of Untagged VLAN100. Would'nt that port always asume that every packet that ingresses on that port would be member of that VLAN? And if there's also a tagged/trunk port on that switch which VLAN100 is a member of, that packet that came in on the untagged port, would get an 801.1q tag and sent out the trunk/tagged port
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PacketTracer

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Re: difference between untagged VLAN and PVID
« Reply #5 on: November 28, 2016, 01:44:18 PM »

Yes, there is a different vendor specific view of how to handle VLANs within Cisco and D-Link switches.

This is how both worlds translate to each other:

Cisco | D-Link
-------------------------------------------------------------------------------------- + --------------------------------------------------------------------------------------------------
access port: "switchport mode access" + "switch port access vlan X" | set port to be member of VLAN X untagged (for egress) and
(valid for ingress and egress) | set port's PVID to be X (for ingress) either
------------------------------------------------------------------------------------- + --------------------------------------------------------------------------------------------------
trunk port (VLANs X,Y,...) with native VLAN N: | set port to be member of VLANs X,Y,... tagged and
"switchport mode trunk" + "switchport trunk allowed vlan X,Y,...,N" | set port to be member of VLAN N untagged (for egress) and
+ "switchport trunk native vlan N" | set port's PVID to be N (for ingress) either
------------------------------------------------------------------------------------- + --------------------------------------------------------------------------------------------------
trunk port (VLANs X,X,...), no native VLAN | set port to be member of VLANs X,Y,... tagged and set port's PVID
"switchport mode trunk" + | to be a DUMMY value (for ingress); for other ports don't use this DUMMY
"switchport trunk allowed vlan X,Y,..."  | value for any purpose other than to be a DUMMY PVID (data sink to nowhere)

The strange thing with D-Link (from the perspective of a Cisco switch administrator) is, that D-Link differentiates the direction for untagged traffic: PVID is for untagged ingress traffic to the port, while the port's membership of an untagged VLAN is for egress only. In  principle you could assign different VLANs for both directions but I cannot see the advantage of this feature.
« Last Edit: November 28, 2016, 02:20:38 PM by PacketTracer »
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