Virtual IP Woes!

I wanted to write about a problem I had recently with the virtual IP address of a Cisco WLC. The IP address had been in use for years with no issues. It was addressed in the 12.x.x.x subnet and was actually a real IP address for one of our external websites. I had read in the past the virtual IP address of a wireless controller should never be a real IP address as it may cause issues but it had been configured this way for years and it was left uncorrected. I was a little confused by how the virtual IP worked and thought why fix something that isn’t broke! 😉 This misconfiguration reared its head in a way I would have never guessed.

Before I can jump in and give you the problem and solution, I need to provide some background. In addition to managing the WiFi network, I have the responsibility of managing our fleet of radios across the U.S. There are thousands of radios that need to be managed and maintained. Keeping inventory of these radios and ensuring these devices are in compliance can be a real challenge! Fortunately with the Motorola Solutions Mototrbo line of radios, managing these is greatly simplified using a radio management server and WiFi. The radios automatically connect to their SSID and the radios can be read from or written to as long as they are connected to the wireless network…all from a single pane of glass.

Utilizing WiFi for Radio Management

Each of the Mototrbo radios have a unique radio ID along with a CAI (Common Air Interface) Network and a CAI Group Network value. While they operate in digital mode, the radios communicate with other radios by one of two ways: a group call (talkgroup) or a private call (one-to-one). When the radios make a private call, the radio combines the radio ID with the CAI value, which is defaulted to 12. When combined, this creates an IP address. For example, radio ID 1 combined with its default CAI of 12 will be 12.0.0.1. You can actually ping radios from your computer! Please note this is not the WiFi address of the radio; it is the radio’s CAI or the main radio interface used to communicate with other radios over VHF, UHF, or 800/900 MHz. The radio has a WiFi interface, a USB interface, and its main radio interface (which is the CAI). Traffic coming into the radio via its WiFi or USB interface that is destined for a 12.x.x.x network will be routed out the CAI. Group calls are routed the same way but the difference is that Group calls default to a 225.x.x.x IP address.

Ok, now that you have that background info I can tell you the rest of the story. One day I received a call stating the Mototrbo radios were not operating correctly. They were very intermittent. Upon investigation it was noted the repeaters seemed to be receiving and retransmitting intermittently and sometimes it was quite often. The random transmissions would come and go with a high enough duty cycle preventing all valid two-way handheld transmissions. It was a big problem. The radio dealer was called out and they investigated and reported back they thought the problem arised when the radios were connected to the WiFi. Honestly I thought it was a bug in the radios but by carefully observing the radios when they connected it was found that when the radios received traffic from the virtual IP address of the WLC (think DHCP on the WLC), the radios simply tried responding to the DHCP server but instead routed the response out its CAI! The result was a bunch of digital traffic being burst out and being repeated on the repeaters taking up airtime.

Radio Routing Traffic Destined to 12.x.x.x Out the CAI

Once it was understood what was happening, the fix to all of this was to change the virtual IP address on the WLC. We changed it to 192.0.1.2 and the problem went away. I will post another article on the virtual IP and how it is used with the captive portal. This topic always confused me a bit so I hope I can help somebody out there that is still fuzzy on this topic. Thanks for reading!

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