2016-02-01-how-to-enrage-your-cable-modem.html

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<p>I have an <a href="http://www.amazon.com/ARRIS-SURFboard-SB6183-DOCSIS-Cable/dp/B00MA5U1FW">Arris SB6183</a>. While setting up a new (non-consumer) router behind it, I kept managing to trigger a modem protection mechanism. The router’s MAC address would disappear from the modem’s list of client addresses, and the modem would refuse to talk to the router at all.</p>

<p>At first I assumed that I’d accidentally set up a DHCP server facing the cable modem, and it was blocking me to prevent pollution of the local broadcast domain.</p>

<pre><code>sudo tcpdump -i &lt;network-interface&gt; port 67 or port 68 -v -e -n
</code></pre>

<p>Just DHCP requests from my router, going unanswered, and broadcast replies to requests from other clients.</p>

<p>Next up: add config settings one by one and see what breaks it.</p>

<p>Turns out, enabling IPv6 causes it reliably. Apparently, Cisco IOS routers begin spewing IPv6 router advertisements when you turn on IPv6. You can <a href="https://www.prolixium.com/mynews?id=804">sniff these</a> (thanks prox). You can also <a href="http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/command/ipv6-cr-book/ipv6-i3.html#wp2583862361">suppress them</a>.</p>

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