The term ‘memory’ is commonly misused when not only talking about router and switch resources, but also with computers and other related devices in general. Regarding Cisco network devices, flash is used to hold the software that runs on the device and the term ‘memory’ is commonly used to describe the RAM.

Before you install a new Cisco IOS Software image on your router, check if your router meets the memory and flash requirements for that image. For this, issue the show version command on your router, and look for these lines:

router#show version

...
cisco RSP4 (R5000) processor with 65536K/2072K bytes of memory
...
16384K bytes of processor board System flash
...

The first line tells you how much Dynamic RAM (DRAM) and Packet memory are installed in your router. Some platforms use a fraction of their DRAM as Packet memory. The memory requirements take this into account, so you have to add both numbers to find the amount of DRAM available on your router (from a memory requirement point of view).

Example 1: Separate DRAM and Packet Memory

...
cisco RSP4 (R5000) processor with 65536K/2072K bytes of memory
...

The 4000, 4500, 4700, and 7500 routers have separate DRAM and Packet memory, so you only need to look at the first number. This shows that the router has 65536 K (or 64 M) of DRAM.

Example 2: Combined DRAM and Packet Memory

...
cisco 2611 (MPC860) processor (revision 0x202) with 29696K/3072K bytes of memory
...

The 1000, 1600, 2500, 2600, 3600, and 7200 routers use a fraction of DRAM as Packet memory, so you need to add both numbers to find out the real amount of DRAM. In this example, the router has 2969 K + 3072 K = 32768 K (or 32 M) of DRAM.

Next, look storage space available for the image:

Example 3: Available Flash Memory

router#show version
...
cisco RSP4 (R5000) processor with 65536K/2072K bytes of memory
...
16384K bytes of processor board System flash
...

The bottom line tells you how much Flash memory is available. Some of it might already be in use. In order to find out the amount of free Flash memory, issue a show flash command:

Router#show flash

System flash directory:
File   Length   Name/status
1    8407884  c2600-io3s56i-mz.121-6.bin
[8407948 bytes used, 8369268 available, 16777216 total]
16384K bytes of processor board System flash (Read/Write)

This shows the current image is using approx half of the current space on the flash.