19 lines
1.3 KiB
Markdown
19 lines
1.3 KiB
Markdown
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I have an [Arris SB6183](http://www.amazon.com/ARRIS-SURFboard-SB6183-DOCSIS-Cable/dp/B00MA5U1FW). 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.
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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.
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sudo tcpdump -i <network-interface> port 67 or port 68 -v -e -n
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Just DHCP requests from my router, going unanswered, and broadcast replies to requests from other clients.
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Next up: add config settings one by one and see what breaks it.
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Turns out, enabling IPv6 causes it reliably. Apparently, Cisco IOS routers begin spewing IPv6 router advertisements when you turn on IPv6. You can [sniff these](https://www.prolixium.com/mynews?id=804) (thanks prox). You can also [suppress them](http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/command/ipv6-cr-book/ipv6-i3.html#wp2583862361).
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