Tuesday, March 26, 2013

A quick note on cable modems and "Serious Switches"

Posting this so someone else might find it and save a day of headbashing.

I have a Ziggo Motorola cable modem which acts as a bridge. Recently, I revamped our home routing infrastructure (because our old 4-port server died), and installed a most excellent HP 1810g switch.

Because this switch supports VLANs, I was able to configure a Raspberry Pi as a "router on a stick" that routed between 3 VLANs, our house LAN, the Ziggo cable and Telfort DSL.

I did note I had to reboot the cable modem a bit to get things to work, but then they did. When the Raspberry Pi was retired from routing & VoIP switching duty, it got replaced by a most excellent HP MicroServer N40L, but try as a might, I couldn't get a DHCP lease through the Ziggo cable modem bridge.

I did see ARP packets come in for other Ziggo IP addresses, but my DHCP requests would never get an answer. I rebooted the modem a few times and performed various other tricks, but nothing helped.

On the internet, some people noted the cable modem would only work for 1 MAC address at a time, so I changed my MAC address to that of the Raspberry, but still no dice.

Yesterday it dawned on me - my fancy switch itself generates LLDP packets! And once the cable modem has seen the switch MAC address, it considers that to be its friend. And thus blocking all my Linux server's DHCP requests!

I turned off LLDP, rebooted the modem, and was back in business.

Moral of this story - your fancy "enterprise" equipment may upset your consumer electronics.