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This chapter is from the book

Essential Network Services: DHCP and NTP

DHCP is a common protocol and is familiar to many network administrators. With DHCP, a scope is defined per subnet and is used to assign IP addresses, along with a subnet mask, from a pool of available addresses. You should assign other values, such as the default gateway and Domain Name System (DNS) server (optional), to the scope by setting option values. Cisco IP phones look for option 150 (multiple servers) or option 66 (single server) from their DHCP server. These options provide the IP address of the TFTP server where the IP phone configuration files are stored. You must configure option 150 with the IP address of the TFTP server for the IP phones to receive their configuration file and successfully boot.

NTP is used to synchronize the date and time on your network devices and for IP telephony to display the time on the IP phone and record the date and time for voicemail messages.

DHCP Services

You can deploy DHCP on any platform that supports customized scope options. These platforms include Windows, Linux, Novell, UNIX, and other operating systems.

When you set up the DHCP service for IP phones, you can define the address pools in the following ways:

  • Single DHCP IP address pool: Define a single DHCP IP address pool if the router is a DHCP server and if you can use a single shared address pool for all your DHCP clients.
  • Separate DHCP IP address pool for each Cisco IP phone: Define a separate pool for each Cisco IP phone if the router is a DHCP server and you need different settings on non–IP phone devices, such as personal computers, on the same subnet.
  • DHCP relay server: Define a DHCP relay server if the router is not a DHCP server and you want to relay DHCP requests from IP phones to a DHCP server on a different subnet.

Listing 7.5 shows router-based DHCP services.

Listing 7.5. Cisco IOS DHCP Configuration

CME(config)# ip dhcp excluded-address
CMERouter(config)# ip dhcp pool mypool
CMERouter(dhcp-config)# network
CMERouter(dhcp-config)# option 150 ip
CMERouter(dhcp-config)# default-router
CMERouter(dhcp-config)# dns-server
CMERouter(dhcp-config)# exit

DHCP Relay Service

Implement DHCP relay when the DHCP server is not on the network where the DHCP clients exist. The Cisco IP phones use a broadcast DHCP request-and-response process, and broadcasts are only valid on the local subnet (VLAN). The solution is to convert and track the DHCP broadcast request to either a unicast or a directed broadcast. The broadcast traverses the network to reach the destination server or subnet and relays the response to the original requesting device. Figure 7.4 shows a router acting as the relay agent, accepting the IP phone broadcasts, obtaining the DHCP information from the server, and forwarding the response to the IP phone.

Figure 7.4

Figure 7.4 DHCP relay services.

For the Cisco router to support DHCP relay services, it must have the DHCP service enabled. The Cisco IOS DHCP server is enabled by default. If it has previously been disabled, use the service dhcp command in global configuration mode to enable this feature.

The ip helper-address ip-address command enables the selective forwarding of certain types of broadcasts, including Bootstrap Protocol (BOOTP) and DHCP. The ip-address parameter specifies the DHCP server to which the messages are forwarded. If you have multiple servers, you can configure one helper address for each server. For the IP phone to obtain IP address information, configure the relay command on each router interface local to the IP phones pointing to the remote DHCP server.

Listing 7.6 shows the configuration commands to implement DHCP relay services.

Listing 7.6. Implementing DHCP Relay Services

CMERouter(config)# service dhcp
CMERouter(config)# interface fastethernet 0/0
CMERouter(config-if)# ip helper-address

This listing configures an IP helper address of on the Fast Ethernet 0/0 (fa0/0) interface. This interface connects to a network that has IP phones but does not have a DHCP server. This command causes the DHCP broadcasts that enter the router on the Fast Ethernet 0/0 interface to be forwarded to DHCP server Each request is tracked by the router so that DHCP responses are forwarded to the correct local Cisco IP phone.

Network Time Protocol

NTP synchronizes network infrastructure (routers and switches) and computers (PCs and servers) to a single clock on the network, known as the clock master. NTP is essential to Cisco Unified Communications deployments.

An NTP network usually gets its time from an authoritative time source. This source can be a radio clock or an atomic clock attached to a time server. NTP then distributes this time across the network. An NTP client initiates a transaction with its server with a polling interval that ranges from 64 to 1,024 seconds. This interval dynamically changes over time, depending on the network conditions between the NTP server and the client. No more than one NTP transaction per minute is needed to synchronize two machines.

One of the strengths of NTP is that it uses Coordinated Universal Time (UTC), which is easily accessed through the GPS satellite system. Because UTC is the same worldwide, networks synchronized to UTC avoid interoperability problems with other networks. This synchronization is particularly important when administrators are troubleshooting IP telephony traffic and need to compare log files from various networks. The time of the internal clock of the Cisco Unified Communications call control platform and the network infrastructure components should be synchronized with an NTP server.

The clock timezone zone hours-offset command sets the time zone and number of hours that the time zone is offset from the UTC (formerly Greenwich Mean Time [GMT]). This command allows the Cisco router to define the operating time zone. If daylight saving time occurs in the area where the system is located, you must configure it using the clock summer-time zone recurring [start-date end-date] command.

Because Cisco Unified Communications call control can run on multiple platforms, the method for enabling NTP varies. To keep the correct time based on the time of a more authoritative source than its own system, perform the following:

  • Cisco IOS router: The ntp server ip-address command configures the Cisco router to synchronize with the NTP server specified by the ip-address parameter.
  • LINUX-based servers: Linux-based versions of Cisco Unified Communications Manager have a web-based interface that you can use to set the NTP server to which they can synchronize.
  • Windows-based servers: Windows-based versions of Cisco Unified Communications Manager can be set from the command line.

Listing 7.7 sets a Cisco router to Pacific Standard Time with daylight saving time configured and synchronizes the router's system time to an NTP server with an IP address of

Listing 7.7. Configuring NTP and Clock Settings

Router(config)# clock timezone PST -8
Router(config)# clock summer-time PDT recurring first sunday april 02:00
   last sunday october 02:00
Router(config)# ntp server
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