discover several important facts:
memory as needed, copies the file, and then verifies that the checksum for the file shows that
no errors occurred in transmission. The show flash command then can be used to verify the
contents of Flash memory (see Example 2-6). (The show flash output can vary between router
families.) Before the new IOS is used, however, the router must be reloaded.
only part of the IOS into RAM, to conserve RAM. Other parts of the IOS file are kept in Flash
memory (Flash memory access time is much slower than RAM). In this case, if Flash memory
must be erased to make room for a new image, the IOS could not continue to run. So, if the
router is running from a portion of the IOS in Flash memory, the router first must be booted
using the IOS in ROM. Then the Flash memory will be in read/write mode, and the erase and
copy processes can be accomplished. The copy tftp flash command in later releases of the IOS
actually performs the entire process for you. In earlier releases, you had to boot the router from
ROM and then issue the copy tftp flash command.
many sources. Two methods are used by a router to determine where it tries to obtain an IOS
image to execute. The first is based on the value of the configuration register, which is a 16-bit
software register in Cisco's more recently developed routers. (Some older routers had a hard-
ware configuration register, with jumpers on the processor card, to set bits to a value of 0 or 1.)
The second method used to determine where the router tries to obtain an IOS image is through
the use of the boot system configuration command. Figure 2-12 shows an example binary
breakdown of the default value for the configuration register.
File Length Name/status
1 6181132 c4500-d-mz.120-5.bin
[4181196 bytes used, 4207412 available, 8388608 total]
8192K bytes of processor board System flash (Read ONLY)