Newer routers, such as the 2500 series, no longer use ROM to store the operating system. Instead, ROM contains a bootstrap program - a tiny OS with just enough code to boot the router and search for the full operating system in flash memory or from other sources. Everything stored here is cleared when the router is powered down or reset. Primary memory contains a running copy of the Cisco IOS, which is loaded into RAM from Flash at startup. In addition, primary memory stores the routing tables, ARP tables and other IOS data structures.
Shared memory is used for buffering.
This is a temporary storage area where packets go when they are waiting to be processed. Flash Memory Flash memory is an erasable ROM, which can be overwritten and reprogrammed. It contains the IOS image file, and can be easily upgraded by 'flashing' a new version onto the chip. Flash memory retains its contents when the router is powered down. It is important to understand the difference between the IOS image file, and the Configuration file.