Tuesday, September 1, 2020

How is the Decision Made for a Device to Join Either the Access or Voice VLAN?

I work for a large enterprise. On our client access switches, the standard port config for a client connection looks something like:

description **Client Access Port*

switchport mode access

switchport access vlan x

switchport voice vlan y

Access ports usually contain end user devices such as PCs, printers, etc. while the voice vlan is dedicated to IP phones only. I notice that when a PC connects to a port with this config, it gets a DHCP address from the access vlan (our data vlan) and if I connect an IP phone to the same port it gets a DHCP address from the voice vlan. Recently, I have worked on a few projects where we have installed various "Cisco Webex SMART kit" specifically these devices:

https://www.cisco.com/c/en/us/products/collateral/collaboration-endpoints/webex-room-series/datasheet-c78-741523.html

When these tablets are connected to the network, they automatically get an IP address in the voice vlan. The person I am working with responsible for provisioning these devices tells me they need to be in the data vlan instead. This is fine, I can change the IP of the device no problem and it should still work on that port. I was just curious as to what the mechanism or logic was to how a device decides which subnet to request a DHCP address for when connected to a port with both an access and voice vlan associated to it. Any insight would be greatly appreciated.



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