I realize this is an old post but I stumbled across it recently and had something to add. Cisco 3600 series IOS 12.4 does have IPv6 support, however you will need to have at least 128M RAM and 32M Flash, best place for that is on EBay. I discovered this when I decied to see what I could do with my 3640 to keep using it and save the money having to replace it. Also check your BOOT ROM version, get the latest. I have not tested this solution yet, so compare the cost to upgrade this router versus the cost to replace it. As the previous post said it been a dead product for 7 years.
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