In my network I have VoIP (VLAN-ID 30) phones and desktop (VLAN-ID 20) computers on separate VLANs. I would like to run a single cable to my desk from the switch that first connects to my VoIP phone, then use the 'output' network port on the VoIP phone to connect a CAT6 cable to my computer.
I am not sure if I am supposed to run VLAN30 over VLAN20, or opposite that. I have tried both but can't get it to work, even when modifying the settings in my phone.
In case it matters, my switch is a Juniper 3900 series with Layer 3 routing capability and my phone has an option to assign VLANs.
11 Answer
You need to setup your switch that the port connected to the phone is tagged for the VOIP traffic (VLAN 30) and untagged for the desktop traffic (VLAN 20). That is the setup that works with most VOIP phones (you must set the phone VLAN option to 30).
However, it might not wise to use this setup. I won't go into details since I don't know your technical background knowledge. But if possible you should use two ports / cables, one to the phone, one to the PC. This is for security (there are some attack scenarios) and performance reasons (the connection from phone to PC has often less performance then a connection between PC and switch).
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