Understanding the network.pdf - Back to Home

ROM is used on the 1600, 2500, 2600, and 3x00 series routers to
handle the router's bootstrap process, and it has just enough
headroom to load the OS. In the case of the 2500, the ROM set also
contains a limited version of the router's operating system, called the
Internetwork Operating System (IOS).
Flash memory is rewritable nonvolatile memory. In the early days of
programmable hardware, floppy disks were used to store boot images
and configuration files, as flash memory was excessively expensive.
Flash is either mounted on the router's motherboard or installed in a
router's PCMCIA slot(s). Flash memory is used on all Cisco routers to
store the IOS. On 4x00 and 7x00 routers, along with the IOS flash, an
additional flash chip known as the bootflash contains the bootstrap OS.
By using the bootflash, the bootstrap application can be upgraded
without a hardware change.
NVRAM is similar to flash in that it does not lose its contents when
power is lost. It is used to store the router's configuration information.
DRAM is used for packet processing and IOS operation. On all router
platforms (except the 7x00 series), DRAM is partitioned into primary
and shared memory. There are two primary memory architectures
used by Cisco routers:
• Read From Flash (RFF)—The Cisco 1600 and 2500 series (and
older) routers use an RFF model. The IOS resides on flash, and
IOS data structures are loaded into RAM as needed.
• Read From RAM (RFR)—The 1600-R, 2600, 36´0, 4´00, and
7x00 series routers are RFR routers. The IOS image is stored in
compressed form on the router's flash. At boot, the IOS's whole
image is decompressed from flash to DRAM where it is accessed
by the router when needed.
Along with the IOS, primary memory is also used to store the router's
configuration information and the data tables needed to process
datagrams. For instance, if IP is being routed, the ARP table and IP
route tables are stored in primary memory. If IPX is in use, the SAP
and IPX route table would be stored. If AppleTalk is routed, the AARP,
Zone Information Table, Name Binding Table, and AppleTalk Route
Table would be stored.
The shared memory is used for datagram processing. By default, the
DRAM is partitioned using a 75/25 split of the router's minimum DRAM
configuration split. On 1600, 2500, and 4x00 series routers, the
primary/shared partition cannot be adjusted. On the 2600 and 36x0