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Chapter 7
Managing a Cisco Internetwork
The Router Boot Sequence
W
hen a router boots up, it performs a series of steps, called the
boot sequence
, to test the hardware and load the necessary software. The boot
sequence consists of the following steps:
1.
The router performs a POST. The POST tests the hardware to verify
that all components of the device are operational and present. For
example, the POST checks for the different interfaces on the router.
The POST is stored in and run from
ROM (read-only memory)
.
2.
The bootstrap looks for and loads the Cisco IOS software. The boot-
strap is a program in ROM that is used to execute programs. The
bootstrap program is responsible for finding where each IOS program
is located and then loading the file. By default, the IOS software is
loaded from flash memory in all Cisco routers.
3.
The IOS software looks for a valid configuration file stored in NVRAM.
This file is called startup-config and is only there if an administrator
copies the running-config file into NVRAM.
4.
If a startup-config file is in NVRAM, the router will load and run
this file. The router is now operational. If a startup-config file is not
in NVRAM, the router will start the setup mode configuration upon
bootup.
Managing Configuration Registers
A
ll Cisco routers have a 16-bit software register, which is written into
NVRAM. By default, the
configuration register
is set to load the Cisco IOS
from
flash memory
and to look for and load the startup-config file from
NVRAM.
Understanding the Configuration Register Bits
The 16 bits of the configuration register are read from 15 to 0, from left to
right. The default configuration setting on Cisco routers is 0x2102. This
means that bits 13, 8, and 1 are on, as shown in Table 7.2. Notice that each
set of four bits is read in binary with a value of 1, 2, 4, and 8, from right to left.
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