Determining Cisco Router Memory Size

The show version EXEC command is able to tell how much Dynamic RAM (DRAM) and packet memory (separate SRAM or shared memory) are installed in a Cisco router.

The Cisco 4000, 4500, 4700, and 7500 series routers have separate DRAM and packet memory. The example below shows a router with 64M of DRAM and 2M of SRAM (packet memory):
cisco RSP4 (R5000) processor with 65536K/2072K bytes of memory.
The example below shows a router with 256M of DRAM and 8M of SRAM (packet memory):
cisco RSP8 (R7000) processor with 262144K/8216K bytes of memory.

The Cisco 1600, 2500, 2600, 3600, and 7200 series routers use a fraction of their DRAM as packet memory. Hence both numbers need to be added to find out the real amount of DRAM. The example below shows a router with 29696K + 3072K = 32768K = 32M of DRAM.
cisco 2611 (MPC860) processor (revision x) with 29696K/3072K bytes of memory.
The example below shows a router with 93184K + 5120K = 98304K = 98M of DRAM.
cisco 2621XM (MPC860P) processor (revision) with 93184K/5120K bytes of memory.

DRAM is mainly used for system processing, eg: storing routing tables, routing protocols data, network accounting information, and running the Cisco IOS software; while packet memory is used for packet buffering of the router network interfaces and CPU cache memory functions. The packet memory on Cisco 4000, 4500, 4700, and 7500 series routers is the total physical I/O memory (also known as SRAM or Fast Memory); while the packet memory on Cisco 1600, 2500, 2600, 3600, and 7200 series routers is the amount of shared memory, which is a portion of the DRAM.

The terms I/O memory (iomem), shared memory, Fast Memory, and PCI memory are all refer to Packet Memory, which is either a separate physical RAM stick or module, or shared DRAM.

Static RAM (SRAM) is a type of semiconductor memory that does not need to be periodically refreshed as Dynamic RAM (DRAM). Memory refresh is the process of periodically reading information from an area of computer memory, and rewriting the read information back to the same area immediately without any modification. SRAM is still volatile, as the data stored in it is eventually lost when the memory is not powered.

SRAM is more expensive, but faster and far less power hungry when idle (no periodical refresh) compared to DRAM. Additionally, due to a more complex internal structure, SRAM is less dense that DRAM and hence not used for high-capacity, low-cost applications, eg: PC main memory.