You want to boot the router using an alternate configuration.
The following set of commands allows you to automatically load a configuration file located on a remote TFTP server when the router boots:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#service config Router1(config)#boot network tftp Network-auto 172.25.1.1 Router1(config)#boot host tftp Router8-auto 172.25.1.1 Router1(config)#end Router1#
By default, when the router reloads, it will read the configuration information from a file in its NVRAM. Cisco commonly refers to this file as the startup configuration file. However, you can configure the router to load all or part of its configuration from a remote server via TFTP. This feature does not prevent the router from loading its startup configuration from NVRAM. In fact, the router will load its local startup file before proceeding to the TFTP server files.
Uses for this feature vary, although most people who implement it do so because their configuration file has grown too large for their NVRAM to handle. It can also be a useful way of keeping an access list that is shared by a number of routers centralized and up-to-date. We have sometimes used it as a temporary measure when the NVRAM in a router is damaged.
However, we consider this feature to be highly risky and recommend avoiding it in most cases. If the problem is simply one of NVRAM capacity, Recipe 1.4 shows how to compress the startup configuration file to help fit more information into your existing NVRAM. Also, since routers can operate for years without reloading, using this feature to keep your routers up-to-date seems pointless.
If you choose to implement remote configuration despite these cautions, you need to understand how the boot process works. When you enable the service config option, the router attempts to load a network file, then a host file. The router assumes that network files are common to all routers, while the host file contains router-specific information. If it can't find these files, the router will generate the following error messages:
%Error opening tftp://255.255.255.255/network-confg (Timed out) %Error opening tftp://255.255.255.255/cisconet.cfg (Timed out) %Error opening tftp://255.255.255.255/router1-confg (Timed out) %Error opening tftp://255.255.255.255/router1.cfg (Timed out)
Here you can see what happened when we enabled the service config option and reloaded our router, which was called router1. It attempted to load several different files automatically. The first two files have generic network file names. The router then looks for the host file under two different names. It attempts to load these configuration files from IP address 255.255.255.255 by default.
When we add the boot commands, the router looks for the specified files from the appropriate TFTP server. Again, notice the order that the router loaded the files: the network file first, followed by the host file.
Loading Network-auto from 172.25.1.1 (via Ethernet0): ! [OK - 27/4096 bytes] Loading Router1-auto from 172.25.1.1 (via Ethernet0): ! [OK - 71/4096 bytes]
If you do not configure the router to load specific network or host filenames, it will try to load the default files, shown in the trace above. If these files don't exist, the router will pause for a significant amount of time while it tries to find them. When you use this feature, you should always include both a network and a host file to load. If you don't need a network file, you can put a file on the server that contains only the keyword end.
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The show version tells you whether the router was able to load these files successfully:
Router1#show version Cisco Internetwork Operating System Software IOS (tm) 2500 Software (C2500-IO-L), Version 12.2(7a), RELEASE SOFTWARE (fc2) Copyright (c) 1986-2002 by cisco Systems, Inc. Compiled Thu 21-Feb-02 02:07 by pwade Image text-base: 0x0304CF80, data-base: 0x00001000 ROM: System Bootstrap, Version 5.2(8a), RELEASE SOFTWARE BOOTLDR: 3000 Bootstrap Software (IGS-RXBOOT), Version 10.2(8a), RELEASE SOFTWARE (fc1) Router1 uptime is 4 minutes System returned to ROM by reload System image file is "flash:c2500-io-l.122-7a.bin" Host configuration file is "tftp://172.25.1.1/Router1-auto" Network configuration file is "tftp://172.25.1.1/Network-auto" cisco 2500 (68030) processor (revision D) with 16384K/2048K bytes of memory. Processor board ID 04915359, with hardware revision 00000000 Bridging software. X.25 software, Version 3.0.0. 2 Ethernet/IEEE 802.3 interface(s) 2 Serial network interface(s) 32K bytes of non-volatile configuration memory. 16384K bytes of processor board System flash (Read ONLY) Configuration register is 0x2102
The service config option is disabled by default. However, if the router tries to boot but cannot find its startup configuration file, it will automatically enable this option and attempt to find a configuration file through the network:
00:00:25: AUTOINSTALL: Ethernet0 is assigned 172.25.1.30 00:00:25: AUTOINSTALL: Obtain siaddr 172.25.1.3 (as config server) 00:00:25: AUTOINSTALL: Obtain default router (opt 3) 172.25.1.3 %Error opening tftp://172.25.1.3/network-confg (No such file or directory) %Error opening tftp://172.25.1.3/cisconet.cfg (No such file or directory) %Error opening tftp://172.25.1.3/router-confg (No such file or directory) %Error opening tftp://172.25.1.3/ciscortr.cfg (No such file or directory) %Error opening tftp://172.25.1.3/network-confg (No such file or directory) %Error opening tftp://172.25.1.3/cisconet.cfg (No such file or directory)
Two interesting things happen if you reload a router with an empty configuration file. First, the router enables its autoinstall option and attempts to acquire an IP address via DHCP. In this example, the router obtained a DHCP address of 172.25.1.30. Second, after it obtains a dynamic address, it attempts to load a configuration file via TFTP.
Notice the filenames that the router cycles through in an attempt to load a configuration file. If there happens to be a file with one of these names in the TFTP directory, the router will download it and use its contents to configure itself. This can cause serious problems.
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