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My Cisco DPC3825 is dated June 2012, recently it has been problematic. How can I update firmware?
Your orginal DPC3825, which is a gateway with Wifi. The SB6121 is a modem without Wifi. Do you have your own router? That would be something you would need to consider before switching.
Also, how much did you pay for the SB6121? It see its about 75$ at Walmart. If your OK with buying online the Netgear CM400 is 65$ on Newegg and is a much newer modem with 8x4 tuner, compared to the SB6121 4x4. Its also available at Walmart too for the same price.