IP Telephony Features
What the IP Telephony Features domain tests
This domain tests voice VLANs, LLDP-MED, Power over Ethernet, and QoS treatment for IP phones on Junos switches. You must know how a phone learns its voice VLAN and how the switch prioritizes signaling and media.
Core rules
- LLDP-MED network policy advertises the voice VLAN ID, 802.1p priority, and DSCP to the phone.
- A voice VLAN can be carried as tagged traffic while the PC behind the phone uses an untagged data VLAN.
- PoE negotiates power class and can be limited per interface; check power budget before enabling phones.
- Voice media is commonly marked EF (DSCP 46); call signaling uses a different class such as AF41.
- DHCP snooping and dynamic ARP inspection protect the access layer, but must be scoped correctly.
Worked example
An IP phone boots on the data VLAN and receives a data-network address. LLDP-MED is disabled on the switch port. The phone has no network-policy TLV, so it does not tag voice traffic with the voice VLAN. Enabling LLDP-MED and defining the voice VLAN in the network policy lets the phone learn the VLAN and 802.1p value. The phone then sends tagged voice frames while the attached PC remains untagged.
Traps
Do not assume PoE is available on every access port. Do not mark voice traffic on the phone only; the switch must trust or classify it. Do not mix voice and data VLAN membership on the same untagged segment. Check LLDP neighbor output before changing VLANs.