Information Technology Reference
In-Depth Information
Fault-Tolerant Boot Strategies
Although network booting is useful, network or server failures can make network booting impossible.
After you have installed and configured the router's Flash memory, configure the boot sequence for the
router to reduce the impact of a server or network failure. The following order is recommended:
1.
Boot an image from Flash memory.
2.
Boot an image using a network boot.
3.
Boot from a ROM image.
The following is an example of how to configure a router with a fault-tolerant boot sequence.
goriot#
configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
goriot(config)#
boot system flash gsxx
goriot(config)#
boot system gsxx 131.108.1.101
goriot(config)#
boot system rom
goriot(config)#
^Z
goriot#
%SYS-5-CONFIG_I: Configured from console by console
goriot#
copy running-config startup-config
[ok]
goriot#
Using this strategy, a router has three sources from which to boot: Flash memory, network boot, and
ROM. Providing alternative sources can help to mitigate any failure of the TFTP server or the network.
The configuration register must be set to allow ROM image booting after failed network
booting attempts. For more information, refer to the hardware configuration manual for
your platform.
Note
Timeouts and Out-of-Order Packets
When network booting, a client might need to retransmit requests before receiving a response to an ARP
request. These retransmissions can result in timeouts and out-of-order packets.
Timeouts (shown as periods in a network booting display) and out-of-order packets (shown as uppercase
O's) do not necessarily prevent a successful network boot. It is acceptable to have either or both timeouts
or out-of-order packets occur during the network boot process.
The following examples show console output from network booting sessions that were successful even
though timeouts and out-of-order packets occurred (exclamation points represent successfully received
packets):
Booting gs3-bfx from 131.108.1.123: !.!!!!!!!!!!!!!!!!!!!!!!
Booting gs3-bfx from 131.108.1.123: !O.O!!!!!!!!!!!!!!!!!!!!!!
If a network boot generates excessive out-of-order packets and timeouts, problems might result. These
problems are discussed later in this chapter, in the section “Booting: Timeouts and Out-of-Order Packets
Prevent Network booting.”
