One big win for systems administration at Anchor is our use of APC Rack Power Distribution Units. These have been documented elsewhere in our blog and wiki but suffice it to say that having remote control over your power ports is a Very Good Thing. Situations where you have servers or other devices with multiple power supply units complicates things slightly, but not that much, especially with the aforementioned Rack PDUs in place.

APCs in particular allow you to configure what are called Multicast Groups. Essentially you tell a couple of the Rack PDUs to talk to each other and share information, and WHAMMO you can turn off and turn on a bunch of ports on separate Rack PDUs simultaneously! So rather than turning off the power to one PSU then rebooting the other, you can conduct a reboot of the power to both PDUs with a single command.

The confusion comes during the configuration of the Multicast Group option. Multicast is a very under-utilised feature of IPv4 (which has now partially been rectified in IPv6), in fact a large chunk of the IPv4 address space is allocated to multicast (and is technically called the Class D space). As with all other portions of IP address-space, this has been carefully portioned into sections and allocated to various purposes. You can see the full list here:

http://www.iana.org/assignments/multicast-addresses/

Being a good sysadmin I consider standards to be of paramount importance, so naturally I wanted to configure our Rack PDUs with multicast addresses suitable for the purpose. There are many existing references on the Internet for how to pick sane and standards-obeying addresses from the multicast range. However, when attempting to follow standards and good reason, I was confronted with this error message:

Multicast IP Address is out of range. Valid values are 224.0.0.3 - 224.0.0.254.

Uh, what? I was under the impression that the range 224.0.0.0/24 was already heavily allocated to entities and purposes other than APC Rack PDUs! So much for following the standard, APC.

http://www.elliottkember.com/ie.html (contains some contextually-appropriate coarse language)

Did you notice the little peel-away flap in the top-left corner?

http://www.youtube.com/watch?v=a0qMe7Z3EYg