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DHCP
Dynamic Host Configuration Protocol (DHCP) is a tool that automates the assignment of IP
addresses. DHCP is a successor of BOOTP, the Bootstrap Protocol. Avaya IP Telephones can
use DHCP to learn their IP addresses, default gateways, call controller, tftp server, QoS
settings, and other parameters.
DHCP is a broadcast protocol. which means that request messages from DHCP clients such as
Avaya IP Telephones are seen by all devices on the local network, but are not forwarded to
additional subnetworks. If the DHCP server is present on a different network, DHCP forwarding
must be enabled on the router. DHCP forwarding converts the broadcast message into a
unicast message, and forwards the message to the configured DHCP server. DHCP forwarding
is offered on most routers and layer 3 switches, including those offered by Avaya and Cisco.
For more information, see
Recommendations for IP Telephony
Avaya recommends using a separate subnetwork for voice. Isolating voice traffic from data
traffic allows protection from viruses, excessive broadcast traffic, and security threats that are
caused by malicious users or external intruders. For most IP Telephony implementations, using
RFC 1918 (private) address space is acceptable. Generally, Voice over IP (VoIP) is not
deployed across the public Internet. Therefore, providing addresses in the private range saves
public IP addresses, and provides a layer of security protection by denying connections directly
in from the Internet. Should a public Internet connection prove necessary, Avaya recommends
setting up a C-LAN and a Media Processor card in a demilitarized zone (DMZ) off the firewall,
and using Communication Manager as a proxy server between the internal and external
networks.
Avaya also recommends using DHCP to configure IP Telephones. Using DHCP reduces
administration to a single point, and reduces the incidence of typographical errors that could
cause configuration problems. Avaya includes special configuration options for IP Telephones in
Option 176. Microsoft and ISC (Linux and Unix) DHCP servers support this option. Additional
methods exist for configuring IP Telephones if a particular DHCP server does not support
Option 176. Contact your Avaya representative for more information.
DHCP / TFTP
or the Avaya IP Telephone LAN Administrator's Guide.
IP addressing
Issue 3.4.1 June 2005
291
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