Flash memory is implemented on a Cisco 2500 using two Single Inline Memory Module (SIMM) slots that hold erasable programmable read-only memory (EPROM). Flash memory is used to store and run the Cisco IOS software. When a system is powered down, the contents of Flash memory are not lost. However, its contents can be upgraded by “flashing” the chip, a concept that we’ll look at in more detail in Chapter 12. While a router is running, the contents of Flash are set to a read-only mode.
Flash memory for a Cisco 2500 series router ranges in size from a minimum of 4MB up to a maximum of 16MB. You might consider adding additional Flash memory to meet the space requirements of the IOS version that you have chosen to run. For a Cisco 2501, the base IP version of IOS 12.0 requires a minimum of 8MB of Flash memory. If your Cisco 2501 shipped with only 4MB of Flash, you would require at least one additional 4MB SIMM. For IOS versions with more advanced feature sets, it is not uncommon to require at least 16MB of Flash.
When installing or upgrading Flash using multiple SIMMs, it is important to note that they must be the same size. For example, if you already have 4MB of Flash and wish to upgrade, you can either replace the 4MB SIMM with an 8MB SIMM, or simply add a second 4MB SIMM. You cannot mix and match SIMMs with different storage capabilities. As such, you cannot have one 4MB and one 8MB SIMM installed at the same time – their storage capabilities must equal.