A phone and a PC on one cable
Offices often have one network socket per desk. The IP phone plugs into it and the PC plugs into a small switch built into the phone. The switch port then carries two VLANs:
- The data VLAN (the access VLAN): the PC's frames, untagged.
- The voice VLAN: the phone's frames, tagged with 802.1Q so they get priority.
The switch tells the phone which voice VLAN to use with CDP (or LLDP-MED), and powers it with PoE. The phone then asks for a DHCP address in the voice VLAN.
In NetLab
▶ SW1 SW1>enable SW1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. SW1(config)#interface fa0/5 SW1(config-if)#switchport mode access SW1(config-if)#switchport access vlan 10 SW1(config-if)#switchport voice vlan 50 SW1(config-if)#spanning-tree portfast %Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc... to this interface when portfast is enabled, can cause temporary bridging loops. Use with CAUTION SW1(config-if)#end *Mar 1 00:00:20.000: %SYS-5-CONFIG_I: Configured from console by console SW1#show interfaces fa0/5 switchport | include Mode|Access|Voice Administrative Mode: static access Operational Mode: static access Access Mode VLAN: 10 (SALES) Trunking Native Mode VLAN: 1 (default) Voice VLAN: 50 (VOICE) Capture Mode Disabled SW1#show cdp neighbors Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone, D - Remote, C - CVTA, M - Two-port Mac Relay Device ID Local Intrfce Holdtme Capability Platform Port ID SEP001A9B146608 Fas 0/5 163 H P M NL-7800 Por 1 R1 Gig 0/1 163 R B S I NL-R3000 Gig 0/0 Total cdp entries displayed : 2 ▶ PH1 IP phone SEP001A9B146608 Network : connected Voice VLAN : 50 IP address : 192.168.50.10/24 TFTP/Gateway : 192.168.50.1 Type status to refresh this screen. PH1> status IP phone SEP001A9B146608 Network : connected Voice VLAN : 50 IP address : 192.168.50.10/24 TFTP/Gateway : 192.168.50.1 Type status to refresh this screen.
A port with a voice VLAN is still an access port: it carries exactly one data VLAN and one voice VLAN, not a trunk. show interfaces trunk doesn't list it.
Tip: Voice traffic is marked for priority (CoS 5 in the tag, DSCP EF in the IP header) so the switches and routers send it first. QoS makes the rest of the plan; phones just need their own VLAN to start.