Enterprise QoS Solution Reference Network Design Guide

Chapter 2 Campus QoS Design
Cisco IP Phones
Version 3.3
Enterprise QoS Solution Reference Network Design Guide
QoS Design Overview
One solution is to place a call to the networking help desk when the move is scheduled, so that the switch
ports can be reconfigured to trust/untrust the endpoints as required. However, this approach dampens the
mobility business advantage of IP telephony, since manual network administration is then be required to
complete the move.
Another solution is to have an intelligent exchange of information between the switch and the devices
plugged into their ports. If the switch discovers a device that is trustworthy, then it can extend trust to it
dynamically; if not, then not.
Cisco IP Phones use the latter solution. In the current Cisco implementation, the intelligent exchange of
information is performed using Cisco Discovery Protocol (CDP).
Figure 2-6 shows a conditional trust boundary extension granted to an IP Phone that has passed a CDP
exchange.
Figure 2-6 Conditionally-Trusted Endpoint—Trust Boundary Extension and Operation
4
1
“CoS 5 = DSCP 46”
“CoS 3 = DSCP 24”
“CoS 0 = DSCP 0”
“I see you’re an IP phone.”
So I will trust your CoS.”
Phone VLAN = 110
“Voice = 5, Signaling = 3”
All PC traffic is reset to CoS 0.
1 Switch and phone exchange CDP; trust boundary is extended to IP phone.
2 Phone sets CoS to 5 for VoIP and to 3 for Call-Signaling traffic.
3 Phone rewrites CoS from PC Port to 0.
4
Switch trusts CoS from phone and maps CoS DSCP for output queuing.
The sequence shown in Figure 2-6 is the following:
1. Switch and phone exchange CDP; trust boundary is extended to IP Phone.
2. Phone sets CoS to 5 for VoIP and to 3 for call signaling traffic.
3. Phone rewrites CoS from PC to 0.
IP Phone
TRUST BOUNDARY
2
3
PC VLAN = 10
PC sets CoS to 5 for all traffic.
4. Switch trusts CoS from phone and maps CoS to DSCP for output queuing.
CDP is a lightweight, proprietary protocol engineered to perform neighbor discovery. It was never
intended as a security or authentication protocol. Therefore, to improve the security of conditional trust
extension, the next generation of Cisco IP Telephony products will incorporate the use of advanced
protocols to perform authentication.
The following overview of some of the main IP Phones helps to explain their impact on access edge QoS
design.
Cisco 7902G— The 7902G is an entry-level IP phone that addresses the voice¬communication
needs of areas where only a minimal amount of features is required, such as lobbies, hallways, and
break rooms. These phones probably would not be moved. The 7902G has only a single 10BASE-T
Ethernet port on the back of the phone; therefore, there is no hardware support to connect a PC to it.
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