Tag Archives: dhcp

Essential protocols and services that support VoIP #2: DHCP, TFTP and NTP

DHCP – Unless you’re deploying only a few IP phones, you won’t want to configure the endpoints individually. And some phones can’t be configured entirely by hand. (For example, to factory reset and apply new firmware to the Cisco 7906, the only method seems to be to use DHCP, as I describe here.) DHCP provides IP and router information to the device, as well as the following useful options:

  • DNS server list
  • Network time protocol (NTP) server(s), where the device will get accurate time
  • TFTP server, where the device will get firmware and, typically, configuration files
  • Vendor-specific options, such as Cisco’s option 150, which can list multiple TFTP servers

Without DHCP, the aforementioned information would all have to be entered by hand, using the phone’s keypad. On some devices, the IP address information is entered through the keypad and then the phone is further configured using a web or telnet interface.

TFTP – Trivial file transfer protocol has a long history of being used for automatic device configuration. It’s a very simple protocol, neither authenticated nor encrypted, and basically provides commands to upload or download a known file (no “browsing” capability) over UDP. Its simplicity allows it to be embedded into the device so that even without firmware, the loader can use TFTP to get a device image and configurations. The Cisco IP phones also use TFTP occasionally during operation to retrieve ringtone files or background images.

NTP – Network time protocol keeps everything on the VoIP network tightly in sync. Devices that use DHCP for address configuration also get a list of NTP servers as one of the options; servers and other core components have to be manually configured with the addresses of NTP servers. For the phones, NTP is mostly a convenience service that sets the clock that is displayed at the top of the screen. Accurate time on call-processing servers and voicemail is an obvious necessity, especially for correct generation of call-detail (billing) records. Also, when maintaining a cluster of servers and other core devices, troubleshooting problems using time-stamped log files is far easier when the logs are in sync.

Interop 2008 wrap-up

Aside from touring the expo floor, I attended a few more sessions Thursday and then again this morning, gravitating toward the virtualization sessions. As we’re now implementing virtual servers in our machine rooms, a lot of the material was review, but not all. I learned that Microsoft has a free hypervisor offering called Hyper-V. It doesn’t have feature parity yet with VMware but since they’re offering it for free, I would expect VMware to respond by lowering their prices on their infrastructure components. This would be very helpful for us. Having been psyched up by these virtualization sessions, I’m really looking forward to our group getting the first few servers moved to VM and machines turned off.

On the show floor I spent a little extra time talking to network monitoring/management companies and a DNS/DHCP/IP address management company. The latter gave me a blue hat with a cat logo. Guess who? We have honed our DNS and DHCP processes over the years with scripts and mastery of vi but there are better approaches.

I appreciated the breadth of the conference and being able to sample training in various areas. It was an exercise for my brain to be continually switching gears (going from session to session or booth to booth).

PSU DHCP in the news

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Nothing may seem more ordinary and un-newsworthy than DHCP. But when DHCP is not functioning, it ends up being news and disrupting the entire network.

Recently, Nominum, provider of the software Penn State uses for DHCP, published a press release and case study proclaiming 100% DHCP uptime for four years at Penn State and describing our use of DHCP to support VoIP, wireless and other applications.

In large VoIP installations, DHCP, TFTP and possibly some other low-level network services are vital in the provisioning of endpoints. As of today, we have just over 12,600 phones relying on these services.