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In this section, the best practices components for preparing the network to properly support Unified Communications are
explored. Topics covered include the following:
• Voice VLANs
• DHCP
• NTP
• Power over Ethernet
• IP Phone firmware and configuration files
Voice VLANs
VLANs provide a logical separation of Layer 3 traffic and are created at Layer 2 (the network switch). A voice VLAN
(VVLAN, also called an Auxiliary VLAN) is an additional VLAN for the exclusive use of VoIP and video traffic. The
benefits of using a VVLAN include isolation from the broadcast traffic data VLANs, a measure of additional security, and
simpler deployment because you do not have to renumber the IP address scheme of the whole network to add VoIP
endpoints. (Each VLAN is a new, separate subnet.)
Most Cisco IP Phones are actually 3-port switches. The port that connects to the network switch can act as an 802. lq
trunk, allowing both voice and data traffic to be multiplexed in their respective VLANs on the single cable to the network
switch. The second port connects the desktop PC to the phone (and thus to the network over the trunk on the first port),
and the third port is an internal one for the voice traffic generated and received by the phone.
On many Cisco switches, the port connecting the phone does not need to be a trunk; it can be an access port instead. The
switch is capable of sending the VVLAN ID using CDP messages, and the phone then sends frames from itself tagged
with the learned VVLAN ID and forwards frames from the attached PC untagged. These untagged frames will be tagged
with the access VLAN ID configured on the switch port when they are processed by the switch.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
The phone adds a QoS marking to its own frames, using the 802. lq frame header Class of Service (CoS) field. The phone
marks its frames as CoS 5 by default. This is the recommended setting, but it can be modified.
The following is a typical switchport configuration for an attached IP Phone in VVLAN 100 and the PC in VLAN 50:
S w i t c h ( c o n f i g ) # i n t e r f a c e FastEthernet 0/1
S w i t c h ( c o n f i g - i f ) # s w i t c h p o r t mode access
S w i t c h ( c o n f i g - i f ) # s w i t c h p o r t access vlan 50
S w i t c h ( c o n f i g - i f ) # s w i t c h p o r t voice vlan 100
S w i t c h ( c o n f i g - i f ) # s p a n n i n g - t r e e p o r t f a st
DHCP
It is recommended that you use DHCP for IP Phone addressing. Create a separate subnet for the Voice VLAN and add the
Option 150 parameter to identify the TFTP server IP address. This can be done on an existing DHCP server, or a new one
can be added if necessary; Cisco routers have DHCP server capability. The following configuration is a typical example
of router-based DHCP to support IP Phones:
s e r v i c e dhcp
! enables the DHCP s e r v i ce
I
ip dhcp excluded-address 10.1.1.1 10.1.1.10
! s p e c i f i e s a s t a r t / e n d range of addresses that DHCP w i l l NOT assign
ip dhcp pool name IP_PH0NES
! Creates a pool of addresses ( c a s e - s e n s i t i v e name) and enters DHCP c o n f i g u r a t i o n mode
I
network 10.1.1.0 255.255.255.0
! Defines the subnet of addresses f o r the pool
d e f a u l t - r o u t e r address 10.1.1.1
! Defines the d e f a u l t gateway
dns-server address 192.168.1.10 192.168.1.11
! i d e n t i f i e s the DNS server IP address(es) - up to 8 I P 's
!
o p t i o n 150 ip 192.168.1.2
! i d e n t i f i e s the TFTP server IP
If you choose to use a DHCP server that resides on a different network, you will need to add the ip helper-address
<ip-address> command on the Voice VLAN interface of the router so that it will forward DHCP broadcasts from the
phones to the DHCP server.
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