RV320 - Why can a port only have one untagged and multiple tagged?

[Chok Ho]

Hi,

I have an RV320 and is trying to setup VLAN.  However, I think my concepts are really bad and would appreciate some help.

If there's anyone kind enough to spend some time explaining.  What does this configuration mean:

Port 1 - VLAN 1 (Tagged), VLAN 25 (untagged), VLAN 100 (Tagged)

Some questions that immediately come to mind:

• does this mean port 1 is a trunk port?  
• what does it mean to say VLAN 1 is tagged and VLAN 100 is also tagged?
• how does the switch know whether the device connected to this port should belong to VLAN 1, 25, or 100 if the port can be tagged and untagged at the same time?

Thanks a lot in advanced for the explanation.

[cchamorr]

Hello and the ask you for reaching out to us.

I see you want to get a better understanding of how VLANS and tagging works, I will do my best to explain it. Before I do so, please understand that I will explain based on our small business devices.

1- A trunk port is a port where you will allow several VLANS.

2- On our small business units, every time you use a port as trunk, you will have to mark one of the VLANS, and only one, as untagged, then any other VLANS will be tagged.

3- You will use a trunk port when you are connecting a device that will also support multiple VLANS, for instance, another switch or an access point. If you are connecting a single device that has only a single VLAN then you need to make sure you are selecting that VLAN as untagged on that port.

4- It doesn't matter that multiple VLANS are untagged on the same port as this only means that those VLANS are all passing on that same port.

5- Lastly, if you have a port configured with multiple VLANS, and you connect a device to it, said device will normally get assigned to the UNTAGGED VLAN.

I hope this helps. Please let us know if you have any other questions.

[Chok Ho]

Thanks!  Though I have a couple of follow-up questions still.  Can you help me with it please?  Many thanks again!

2- On our small business units, every time you use a port as trunk, you will have to mark one of the VLANS, and only one, as untagged, then any other VLANS will be tagged.

Why is it that only one VLAN can be marked as untagged?  From my understanding, untagged ports connect to end devices (that aren't VLAN aware).  In that case, why can't we have two untagged VLAN?

4- It doesn't matter that multiple VLANS are untagged on the same port as this only means that those VLANS are all passing on that same port.

Could you elaborate on this?  I really don't understand why a certain port can be tagged and untagged at the same time.  What then does this achieve?

5- Lastly, if you have a port configured with multiple VLANS, and you connect a device to it, said device will normally get assigned to the UNTAGGED VLAN.

This is probably because the device isn't VLAN-aware right?  if a VLAN-aware device is connected to it, does it require additional configuration to recognise the VLAN (since trunk port carry multiple VLAN)?  I have another Cisco SG200-08 switch connected to RV320.

[cchamorr · Accepted solution]

Hello, 

I will be happy to help

2- Because if you had more than one untagged VLAN on a port, how would the devices that are not VLAN aware know which of the two untagged VLANS to get connected to?

4- Remember, untagged VLANS go to end devices that are not VLAN aware, but there are scenarios where you will need to pass more than one VLAN on a port, for instance, when you have VoIP phones and you are able to connect a PC directly to the phone and just the phone connects to the switch, you will need for that single port to pass the info for the VoIP VLAN as well as the data VLAN for the PC, in that case, you will mark the VLAN going to the PC as untagged and the VLAN going to the phone as tagged.

5- yes, that's right, if you have VLAN capable devices and you connec them. To a trunk port with multiple VLANS on it, the device will get connected to the VLAN that matches its tag. In the case where you have a router with multiple VLANS and you are also using a switch to carry those VLANS along, you will need to configure the uplink ports between the switch and the router as trunk, with the main VLAN as untagged and any other VLAN as tagged the configuration on the router port needs to match exactly the configuration on the switch port.

i hope this helps.

[boayers]

Hello let me elaborate a bit on your questions.

While it is possible to have more than one untagged VLAN on a port, this is typically not done. It's easy for a switch to egress multiple VLANs untagged, it would not know what to do with the ingress traffic as the VLAN it originated from would be indeterminable. 

Tagging traffic is how you tell devices on the other end which VLAN the traffic should belong to. When traffic is untagged a non VLAN-aware device such as a PC will typically default to the native VLAN. You are correct that a VLAN-aware device such as the SG200-08 will need additional configuration. Any ports on the switch you wish to use for specific VLAN traffic must be tagged for those VLANs. This document from the Cisco Small Business Knowledge Base may be of use to you in configuring this. 

http://sbkb.cisco.com/CiscoSB/GetArticle.aspx?docid=67844b99e2da4a7f88db0c588197487d_Creating_VLANs_on_Cisco_Managed_Switches.xml&pid=2&converted=0

I hope I've been able to answer your questions. Feel free to let us know if you need more help.