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Old 08-17-2011, 10:14 PM   #1 (permalink)
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Default Power Cycling - Router/Modem, or Modem/Router?

So I just had an issue where the router was not pulling an address properly from the modem. This is with Comcast cable.

Traditionally I unplug both devices. I power on the modem and give it 60 seconds. Once done, I plug in the router. In no less than 400 situations prior, this has worked beautifully. However, this time it didn't do the job. What fixed it? Doing it the OPPOSITE way. Letting the router be turned on first, then turning on the modem. This action allowed everything to work flawlessly.

So, why is that? What is it that the router and modem are doing as far as communication goes that dictates things go in one direction or the other?

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Old 08-17-2011, 10:53 PM   #2 (permalink)
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Default Re: Power Cycling - Router/Modem, or Modem/Router?

It should work either way tbh. When you switch your modem on it'll usually send out a DHCP request to your ISP's router. It'll also usually start up its own DHCP service and wait for incoming DHCP requests from your router, or it'll be ready to forward DHCP requests on to its assigned primary DHCP server.

If you turn on your router without turning on your modem first and it sends out a DHCP query, it'll obviously remain unanswered and it'll try again after a bit.
Probably best though to do what you were doing. Switch modem on first, wait till it acquires an IP, then switch the router on. Weird that it didn't work like that. Only thing I can think of is maybe one/both of the devices didn't reset properly when you switched them off?

Install Wireshark on your PC and take a look at the network traffic, filter just for DHCP. It's pretty cool
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