I just started a new job and part of it is to manage and maintain the ICT. As the previous network/system engineer left the company i'm on my own to figure out how everything works.

I have experience with basic networking and i know what vlans are but never worked with them. I don't understand how the vlan configuration that i "found" here works. I watched a lot of youtube video's, read some articles on the web and read the manual of the switches but i still have multiple questions about this.

They use (6) LANCOM ES-2126 switches and Tag-Based vlans. There are Tag-based groups configured on the switches and some ports are member to multiple vlans. If i understand this well, this means that 1 port receives traffic from multiple vlans? (so this are trunk ports?) Is this correct?

I dont understand why devices like phones and computers are connected to these ports as i understood from my research that trunk ports are meant to link switches and routers so you don't need a wire for each vlan. Is it a good idea to connect devices like phones and computers to these a trunk?? ports?

Another thing i don't understand is that in some Tag-based VLAN groups the Untag is applied to some ports. When i understand this function correct, it removes the vlan tag (VID of the concerning Tag-based vlan group) from the package before it leaves the port? is this correct? and why should the tag be removed?

When a device (e.g.) computer sends information to antoher device, which vlan tag do these packages get? Is this the PVID i configured for the concerning port the devices is connected to? Or does this have also a relation with the Tag-bases groups?

I also read something that tag based vlans are using MAC addresses to add tags to packages but i cant find VID-MAC relations anywhere. Is it possible that this network works without and just bases VID on something else?

Sorry when this are "stupid" questions but i have no idea how to get answers on these questions as no-one knows anything about the network and the old engineer is not available for helping me out.

1 Answer 1


I guess the part you are missing is that VoIP phones have built-in internal switches. This allows the VoIP phone to be on one VLAN and a PC attached to the VoIP phone to be on a different VLAN.

Some switch vendors implement CDP or LLDP to allow the phone to negotiate a trunk, while others do not, so they require that a trunk be configured. A PC connecting to the trunk interface will simply use the native (untagged) VLAN on the trunk, while a VoIP phone will used the tagged VLAN, passing the native VLAN on through to the built-in interface for a PC connected to the phone.

  • I understand what you are saying about the build in switches but the computers are not connected to the network via the IP phones, they use their own patchpoints. The port where they are connected to on the switch is sometimes member of 2 (or more) Vlan groups -- Is it always that a PC "ignores" vlans and use the native vlan when they are connected to a trunk port? In that case it they use the PVID as vlan?
    – CodeNinja
    Apr 7, 2020 at 14:37
  • 1
    "Is it always that a PC "ignores" vlans and use the native vlan when they are connected to a trunk port?" That is what I explained. Most end-devices do not understand tagged frames, and they will drop them as bad frames. The native (untagged) frames will be used.
    – Ron Maupin
    Apr 7, 2020 at 14:40

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