When a device is plugged into a switch (let's say, a Cisco Catalyst switch) how does a switch know whether the device is a VoIP one - and subject to being placed into the voice VLAN - or some other kind, i.e., a data device, and this subject to being placed into the data VLAN?

I know the question may sound dumb but this is the best way I can ask it.




Generally it is either 1 of 2 ways: Either the phone is getting it's configuration information (including VLAN information) from some source (DHCP option/config download/etc) or the switch is using LLDP (Link Layer Discovery Protocol) to discover it's a phone because the phone is advertising with LLDP that it is a phone.

  • Thanks. I'd say #2 makes sense. I will look into that further. #1, however, sounds a bit questionable to me. The phone can only get IP-configured (whether via DHCP or using someother mechanism) once it has been placed into a VLAN. Now prior to it being plugged in the port is down on the switch. After it's been plugged in, given that depending upon what device it is, the switch puts it in its appropriate VLAN it seems quite reasonable to ask, how does it (the switch) know whether the device that has just been plugged in is a VoIP device or not? – Boris Epstein Apr 15 '15 at 16:25
  • The DHCP options are set for whatever scope the devices will come up on. The phones pick up the device specific options, set the config on the phone accordingly and usually restart on the new VLAN with the updated config. – Rex Jun 18 '15 at 15:58

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