1.3 Memory Details of a Typical Cisco 25xx Series Routers
1.3.1 DRAM
This memory is analogous to the memory in PCs. DRAM is a fast memory and loses its contents when the system is restarted (power cycled).
The DRAM has Main Processor Memory and Shared Input/Output (I/O) memory.
Main Processor Memory is used to hold routing tables, fast switching cache, running configurations, and so on. It can take unused shared I/O memory, if needed.
Shared I/O memory is used for temporary storage of packets in system buffers.
1.3.2 Flash
Flash memory is typically located on a processor board SIMM card. Flash is a permanent storage for the Cisco IOS software image, backup configurations, and any other files. Cisco IOS software is run from the flash memory, these image files are defined as relocatable, because the Cisco IOS software image can be executed from different locations in the Flash.
1.3.3 NVRAM
NVRAM is a Non-Volatile RAM used as permanent storage and this memory is re-writable. NVRAM is used to store the startup configuration. This is the configuration file that IOS reads after the router OS is loaded and running. It is a fast non volatile memory and does not lose contents when the router is switched off.
1.3.4 BOOT ROM
This refers to erasable programmable read-only memory (EPROM) and is used to permanently store the startup diagnostic code (ROM Monitor), and RxBoot. Boot ROM size is 2 MB. This type of memory consists of one or more memory chips on a router's processor board. This code runs when the router starts.
