HP Procurve MSM422 / MAP-625 clients flow the dhcp server [solved]

• Stefan Helander


• 31 May 2017

A client was using the HP Procurve MSM422 / MAP-625 MultiService Access Point (wifi). It was being used in a rather crowded wifi environment and the problem was the wifi clients keept reconnecting and renegotioating so often that the DHCP server was overflowed, sometimes with DHCP requests every few seconds. They had been struggling with the problem for a couple of months and the focus was aimed at the DHCP requests.

It turned out that the problem was not in the DHCP negotiation at all, but was caused by the wireless clients that keept losing connection and each time when they reconnected a DHCP request was sent.

The client was misinformed, that 5 GHz band was prohibited in the country where it was set up (which is acutally not). So both radios (radio 1 and 2) where set to 2,4 GHz where radio 1 was set to 802.11n/b/g and radio 2 with 802.11b/g.

Furthermore, for radio 2 the value of the Antenna gain was set to the maximum which is 29 dBi in an attempt to boost the maximum power out of the system. However, this field works the opposite. In order to be regulatory compliant and not emit more than the allowed power, this field informs the system that it is connected to an antenna with 29 dBi gain, so to not emit illegal levels of power (output power + antenna gain), the system will reduce the actual output power (i.e. the power input to the antenna) by 29 dBi. But the system was using the internal antennas which, I guess, has more or less no antenna gain. This caused the system to actually emit -29 dBi, i.e. a very weak wifi signal.

The low power output made it hard for the wifi clients to “hear” the access point, which caused them constantly to lose connection and when reconnecting, they were sending a DHCP request, hogging down the DHCP server.

Solution:

• Radio 1 was configured to use the 5 GHz band. This band is much less crowded than the 2,4 GHz band and the bandwidth is better, so when a client has the possibility, it is preferred if it can use the 5 GHz band.
• Radio 2 was configured to use Internal Antenna with 0 dBi antenna gain.

Settings used when the problem was solved like this (click on the image to enhance it):