3.2 NETWORK TIME PROTOCOL (NTP)

23

this can provide a real last-resort access method to log into a device to find out if it has failed or if it has been isolated from the network by multiple link failures. If there is a terminal server in the hub, then it is probably simpler to connect the modem to the terminal server’s aux port.

3.1.4REMOTE POWER MANAGEMENT

In the worst case, if all else fails, it may be necessary to power cycle the device in order to recover service. If the device happens to be in the same building as your office, then walking through to reboot the device is a simple task. However, if the device is in an unmanned location hundreds of miles from your office, then it is going to be really problematic to recover quickly. One device that can make life a whole lot easier is a remote power device. It permits operators to interrupt and reconnect the power to a network device from a remote location. There are a variety of these devices on the market, access to which is provided via a console port or using the OOB network to connect using telnet, SSH and/or SNMP (see below).

Note: cutting and restarting the power is generally acceptable for network devices (routers, switches, etc.). However, servers (UNIX servers in particular) tend to react badly to being rebooted without a clean shutdown. This is particularly bad because of disk caching mechanisms. By cutting the power without first flushing the caches, you can end up with your disks in an indeterminate state. In the worst case, this can prevent the server from rebooting. This leads to the related issue of uninterruptible power supplies (UPS).

3.1.5UNINTERRUPTIBLE POWER SUPPLIES (UPS)

The loss of power to a network device is a pretty catastrophic event. Therefore, it is advisable to use a UPS. As the name suggests, a UPS provides a source of power in the event of the failure of the main power supply. Unless you have a room full of batteries, it is unlikely that a UPS will actually be able to sustain power-hungry network devices for more than a few minutes. However, this gives time for an alternative source to kick in (e.g. a generator) without having to reboot each box (with the attendant spike in power draw as all devices power up simultaneously). If there is no alternative power source, the UPS will give the operator time to gracefully shut down servers so that they reboot cleanly.

3.2 NETWORK TIME PROTOCOL (NTP)

NTP is very important to the scalable operation of a network. In particular, it is essential that if an event affecting multiple devices occurs, it is possible to correlate the logs from each of the affected devices to identify simultaneous events. The inability to correlate these events can make it significantly more difficult to identify, isolate and fix any faults. NTP allows individual devices to synchronize their internal clocks so that all network devices report the same time to within a few milliseconds.