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Booting Router From TFTP Server

R

Rubicone

Senior member
What is the value one sets in the configuration register to boot from a TFTP server on Cisco 1600/2500 series routers? TIA.
 
0x102 will tell the router to use whatever is configured for the "boot system" line of the config. If there is no "boot system" line, then it will look for a TFTP server and try to load a default image. What do you name this config file? Here is what cisco says:



<< The router forms the default boot filename by starting with the word cisco and then appending the octal equivalent of the boot field number in the configuration register, followed by a hyphen (-) and the processor type name (cisconn-cpu). See the appropriate hardware installation guide for details on the configuration register and default filename >>



Personally I would just set config register as stated above, and specify a &quot;boot system tftp rubicone-cfg x.x.x.x, where rubicone-cfg is the name of the config file and x.x.x.x is the IP of the tftp server. Hope that helps.
 
I don't quite follow you. If I create a configuration file or save a copy of the IOS on a TFTP server does one issue the config register command followed by a specific hex value (eg. ox102), proceeded by the name of the file and IP address of the server? Or does one isssue the boot command in privileged/global config mode?
 
I'm sorry if I caused you some confusion -- I was throwing around the word &quot;config&quot; when I was talking about the IOS software. Maybe I'll wake up later this afternoon.

To clarify -- the config register is something you only have to set once, either from rommon or put it in the startup-config. Then you need to put the boot system command I posted earlier in the startup-config as well. When the router boots and loads the config from NVRAM it will see that it needs to go out to that TFTP host to get the software, specified by the filename you put on the boot system command. Again, sorry for the confusion.
 
I usually don't argue with what CTR states cos he's the reference 🙂 but I believe the command is boot system tftp x.x.x.x filename (just verified that on the cisco book)

Rubicone: 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. The second is through the use of the boot sytem command. So you can use either of them.
 
Dark:

I was getting my info from a page I have bookmarked, probably about a year old. Wouldn't be the first time Cisco's website was wrong!
 
I am reading Cisco's curriculum and there is no reference I could find for a register value in config register for booting off a tftp server. To quote them:

&quot;To change the boot field and leave all other bits set at their original values (initially the config register contains 0x010x), follow these guidelines:

Set the configuration register value to 0x100 if you need to enter the ROM monitor (primarily a programmer's environment). From ROM monitor, boot the operating system manually by using the b command at the ROM monitor prompt. (This value sets the boot field bits to 0-0-0-0.)
Set the configuration register to 0x101 to configure the system to boot automatically from ROM. (This value sets the boot field bits to 0-0-0-1.)
Set the configuration register to any value from 0x102 to 0x10F to configure the system to use the boot system commands in NVRAM. This is the default. (These values set the boot field bits to 0-0-1-0 through 1-1-1-1.)&quot;

So does this mean I have to use the boot command to get the router to boot off the TFTP server? Or does one of these settings automatically seek the IOS in a TFTP server when not found in either Flash, NVRAM. I am trying to set the first place that is sought the TFTP server.
 
Setting the LSB (hex) of the config register to anything between 2 and F tells the router to boot the system from whatever is specified by the &quot;boot system&quot; command in the startup config.
 
Hmm update, I just logged to a router to try and it's boot sytem tftp filename x.x.x.x this is under 12.x. Maybe the infos on the book are for another OS. Well that proves what we all know: CTR is DA MAN as USUAL 😀
 
I believe (it's been a while since class) that the config register (normally 0x102? something like that)tells the router (bootstrap code/monitor)to boot using BOOTP, once it get's it's config info (IP address, mask, server address), it then does a directed TFPT to get the image.

Since BOOTP is a broadcast-oriented request, any intermediate router must be configured with what Cisco calls &quot;IP Helper.&quot; IP Helper converts the broadcast into a unicast aimed at the specified resource (IP Helper by default also passes/converts a bunch of other broadcast-related requests).

I'm a little fuzzy on it, but that's the way I remember it.

FWIW

Scott
 
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