Note: this was written in 2012 and the technical details reflect that time.
As a contractor, you need to keep your skills up, so I kept a lab at home. The clients I dealt with were mostly small and medium sized businesses. SMBs don't generally have large networks and I would often use the lab to try out ideas.
I also liked to browse eBay for cheap gear. If you have looked at the Cisco 1841 router, you would see that many are advertised as having IOS 15, but beware; they often don't have enough memory to run. IOS12 will run on 128MB but IOS15 requires the full 384MB in order to run correctly.
This has happened to me twice now; twice I have purchased 1841's with IOS 15, and twice they have not had the required amount of memory to run IOS 15.
My first 1841 came with IOS12 and 15 so I just dropped back to 12. This is currently my home router. My latest purchase needs to run IOS15 for my lab work, so I decided to try a memory upgrade.
This is the Cisco page showing how to add the additional memory. It is very straight forward requiring only the removal of one screw to get the box open.
Of course you can't just throw any old memory in there. The 1841 comes with either 128MB or 256MB of DRAM, with a single slot for upgrading to 384MB. In my case I have 128MB so I needed a 256MB ram module, and after a little research, This is the one to get.
Installation took maybe all of 5 minutes, and the router is up and running again:
And that is all there is to it. I've been working with the router most of the day, and so far run into no issues with functionality or with the extra memory.