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Home News IP: networks, access, QoS "Storm Control", a Tricky Switch Feature
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"Storm Control", a Tricky Switch Feature

Setting up my home network in my new home in Berlin, it was time to deploy the managed 16 port switch to accommodate all Ethernet connections. It has proven to be beneficial to connect every device using an Ethernet cable instead of the often available WLAN. This is especially valid if there is video data to transfer and the WLAN AP is so far away that the maximum speed is not available. Furthermore I like to be able to isolate single ports and use the port mirror option to monitor the traffic of a host on a specific switch port.

Anyway, after I switched from my temporary dumb 8 port switch to my managed 16 port switch, I noticed that my network printer became unreliable and refused to print. How is this possible?

My first investigation showed that I was able to ping the printer's IP address. Then it was Wireshark time. No reply to the MDNS queries from my Desktop. MDNS is used to find the IP address of a host in a local network without using centrally administered DNS servers. The originating host sends a multicast packet to all concerned hosts on the LAN, e.g. asking for the IP address of "printer.local". The printer would answer with its IP address. But there was no answer visible on the desktop which was about to use the printer. Playing around, I pinged the multicast IP for MDNS, which is 224.0.0.251. I knew that there are a couple of hosts using MDNS, and they all should reply, namely the Samsung printer, the Apple TV box and the AV receiver. Interestingly, there was only one answer visible on Wireshark, which was the Apple TV. Removing this device, I could see now the AV receiver answering my ping. To make a long story short, actually all hosts answered my ping request, but the "smart switch" blocked all succeeding answers after it got a first multicast answer, they never reached the requesting host. The reason is the feature "storm control" which was activated for multicast and broadcast packets. I don't know how exactly it works, but seeing 3 multicast packets (the answers to my ping) enter the switch simultaneously, it kind of panicked and killed 2 of them in an effort to protect my network form being flooded with multicast packets.

After I disabled this feature, I could see all 3 MDNS supporting hosts answer simultaneously my ping request. Also the printer was working without any problem.  I do understand that the switch starts to drop packets as soon as it sees multicast packets entering the switch simultaneously (e.g. my ping answers). But there should be just a single answer to a single MDNS query. Why the switch blocks this printer MDNS query while "storm control" is active remains to be further investigated.

Anyway, disabling storm control in the D-Link DGS-1210-16 solved my problem, and maybe yours, in case you had a similar problem and googled for a solution!