Tuesday 11 December 2018

Dealing with a SURFboard Cable Modem (Arris SB6141)

So I had a nice wifi setup that ticked along for a couple of months with no complaints.  It was a bit unusual, but nothing terrifically so.

Shaw communications had supplied a SURFboard cable modem, which fed into a Linksys EA9200 router, which then supplied wifi to the household, as well as fed into some TP-Link AV500 power adapters to head upstairs and allow for a hard-wired VoIP box to be installed.

After a few months, some new doors were installed, and suddenly the wifi signal was weak and dropping constantly.  The doors were upgraded from glass to solid, fire proof doors (this was a multi-unit house), and so I can only suspect that that was part of the problem.

In any case, I thought maybe I could swap around things slightly, and move the router upstairs into the office of one unit, where it was directly over the main room of the lower unit.

Unfortunately, as soon as I got upstairs, no internet.

Hypothesis - does the powerline adapter somehow not work with the cable modem directly?

Test - plug another inexpensive router into the cable modem first, then the powerline adapter, then the router - kludgy, but one does what one must.

Result - cable modem switched from blue downstream connection light to red, and no internet.

Test Two - plug in my laptop directly to cable modem and see if it's just how the router is configured (even though I did a factory reset on the router).

Result - same as above, red light instead of blue, and no internet.

Bring the original modem back downstairs, plug it in - blue light, and internet.  In fact, that's exactly how it played out.  ONLY the original modem worked with the cable modem, anything else was "blacklisted" with a red light.

Much head scratching and call to tech support (with escalation) finally revealed the awful truth -

The red flashing light on a SURFboard modem does NOT indicate a problem - but rather that you are not using a 1GB capable device, and have switched to 100MB or whatever - not an issue for these cable connections where you're getting from between 5MB and 10MB connections anyway.

Shaw communications cable modem only gives out ONE SINGLE IP ADDRESS via DHCP.  You can only have one device hooked up to it asking for an IP, and then IT has to also be a DHCP server for everything else downstream.

Shaw communications cable modem will only give out a new IP address if it is fully power cycled.

That explains pretty much everything - except my initial problem, which I attribute to me being too impatient with the router, which admittedly takes a few minutes to reboot and connect to the modem at the best of times.

Otherwise - if you try to change any device hooked directly to the modem, you MUST power cycle the modem.  This is not intuitive, as every other router/modem that I'm aware of is easily able to ascertain that a device has been swapped, and re-assign it a new IP.

Long story short - did you try turning it off and on again?

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