Specifications

CHAPTER 8
Voice and Video in a Campus Network
Cisco IP phones have a small internal switch that places an 802.1q tag on voice traffic and marks the Class of Service
(CoS) bits in the tag. Data traffic is sent untagged over the native VLAN. The switch port does not actually become a
trunk and still can be configured as an access port. It is considered a multi-VLAN access port.
Power over Ethernet (PoE) Switches
IP phones can receive power from PoE switches, eliminating the need for an electrical plug.
Two power standards are the Cisco Inline PoE and the IEEE’s 802.3af standard. Both have a method for sensing that a
powered device is connected to the port. 802.3af specifies a method for determining the amount of power needed by the
device. Cisco devices, when connected to Cisco switches, can additionally use CDP to send that information. Most IP
phones require no more than 15 W of power specified in 802.3af, but some new phones, access points, and surveillance
cameras require more. The 802.3at standard will specify up to 30 W of power. Some Cisco switches can currently supply
up to 20 W.
Because a switch assumes 15.4 W of power until the connected device tells it the amount needed (via CDP), calculate
your power budget based on 15.4 W for all devices because if the switch reboots, all ports will ask for 15.4 W until they
get the correct information. Non-CDP devices always get allocated 15.4 W.
Cisco PoE switches automatically detect and provide power. To disable this function, or to reenable it, use the interface
command power inline {never | auto}. To view interfaces and the power allotted to each, use show power inline [inter-
face].
QoS for VoIP
QoS gives special treatment to certain traffic at the expense of others. Using QoS in the network has several advantages:
n Prioritizes access to resources so that critical traffic can be served
n Allows good management of network resources
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CCNP SWITCH 642-813 Quick Reference by Denise Donohue
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