Which Cisco switch feature enables IP phones to be assigned IP addresses from a different subnet than the workstation attached to the same port?
- Auxiliary VLAN
- in-line power
Auxiliary VLANs can assist in the deployment of Cisco IP telephones by allowing a Catalyst switch access port to support the data device on one VLAN and have the IP telephone assigned to a different VLAN. The auxiliary VLAN ID is not required to match the native VLAN associated with the access port of the switch. Therefore, the two port attached devices (the phone and the workstation) can be on two different broadcast domains or IP subnets. Some Cisco Catalyst switches provide the auxiliary VLAN feature that provides the automatic assignment of an IP telephone to a VLAN. This auxiliary VLAN feature is also known as the voice VLAN feature.
IP telephones typically have a built in 3-port 10/100 hub. One port internally attaches to the phone, one port is attached to the switch access port, and the last port is used to connect to the workstation. The switch uses CDP on a port on which an auxiliary VLAN has been configured to advertise 802.1Q and 802.1P information. The attached IP telephone can use the advertised information to learn which VLAN ID and priority tag to use. The default Class of Service (CoS) value for incoming traffic is 0.
Having the telephone and the data device use different frame types and belong to different VLANs allows the administrator to place the telephones on their own subnet. Voice traffic on a separate VLAN is less likely to contend with the data devices.
To configure the interface on the switch to support auxiliary VLANs, use the following commands:
- switchport voice vlan VVID: This command is used to enable the access port to forward 802.1Q packets received tagged with the ID of the voice VLAN (VVID) to the VLAN defined as the voice VLAN. By default, 802.1Q packets are sent by IP phones with a QoS priority of 5.
- switchport voice vlan dot1p: This command is used to instruct the attached IP telephone to send packets using the VLAN ID of the port’s native VLAN with a dot1p priority of 5.
Layer 2 Technologies
Configure and verify VLANs