VoIP (Voce over IP) is a group of technologies that allows for voice calls using IP networks instead of analogue phone lines. Different VoIP services will use different protocols, software, and even hardware.

IP Telephones

Traditional phones operate over the Public Switched Telephone Network (PSTN). (Sometimes this is called POTS (Plain Old Telephone Service). IP Phones, on the other hand, use VoIP tech to enable phone calls over an IP network (such as the Internet).

”IP Phone” typically refers specifically to hardware: a physical telephone. Most IP phones look nearly identical to regular landline telephones, but they will have RJ45 network ports rather than typical phone jacks. A number of companies, including Cisco, produce IP phones.

IP phones need to be connected to a network switch just like any other end host device. IP phones are commonly used by workers at their desks, where there is likely to be a desktop workstation that also needs an Ethernet connection. It would be cumbersome to run two separate cables from the switch to each desk in an office, and that would mean taking double the number of interfaces on the switch for every desk. Furthermore, it is common for workers to use software on their desktop to track and manage phone calls, so the IP phone and the workstation PC, so it makes sense to have them directly connected. This is why many IP phones have a small 3-port switch built-in. One port is the “uplink” and connects to the external switch. Another is the “downlink” which is used to connect to the PC (It could be used for any end-host but a PC is most common). The final port connects internally to the phone itself. This way you only need a single switch interface for each desk. All the traffic from the PC will be pass through the phone’s internal switch.


Why is it important to reduce the number of switch interfaces we use?

Because enterprise switches are expensive, and the more interfaces we use the more switches we need to buy.


IP Telephones are commonly powered with Power over Ethernet (PoE).

Voice VLANs

It’s preferable to separate VoIP traffic from other network traffic using a voice VLAN. This is done to simplify differentiating data for Quality of Service (QoS). For more information on VLANs, see the full page here.

Configuring a voice VLAN is similar to configuring any other VLAN. Configuration is done on the incoming interface of the uplink switch, not on IP phones.

  • (config-if)#switchport mode access
  • (config-if)#switchport access vlan 10
  • (config-if)#switchport voice vlan 11

The switch will use CDP to tell IP phones connected to the interface to tag their data in VLAN 11. All other data that passes through the interface (by passing through the phone’s internal switch) will remain untagged, and will be transmitted through VLAN 10.


Generally, interfaces that accept traffic from multiple VLANs are trunk ports. This is an exception to the rule. An access port may use a voice VLAN in addition to one other VLAN and still be an access port