Setting the serial number on your 2600 series router

From Internetworkpro

There may be times that you need to change the serial number on your 2600 router. Please note that this is unsupported and could be dangerous to your router. I am not responsible if your router fails to boot.

First of all you need to break into your rommon. Reboot the router and send a break sequence right afterthe first text comes on your console. You should now be sitting at a rommon1> prompt.

Type cookie

System Bootstrap, Version 12.2(6r),  RELEASE SOFTWARE (fc1)
TAC Support: http://www.cisco.com/tac
Copyright (c) 2001 by cisco Systems, Inc.
PC = 0xfff0ac0c, Vector = 0x500, SP = 0x68012748
C2600 platform with 49152 Kbytes of main memory

PC = 0xfff0ac0c, Vector = 0x500, SP = 0x800046ac

monitor: command "boot" aborted due to user interrupt
rommon 1 > cookie
cookie:
01 01 00 09 43 ab 25 60 43 20 00 ff 00 91 00 20
00 00 00 00 00 00 00 00 46 54 58 09 45 57 30 4d
59 02 00 00 00 00 00 00 00 ff ff ff 50 06 49 0b
17 0f ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff

The first 5 words in the cookie represent the priv password. To decode the password use this website: http://www.bitshift.ch/eng/support/kbase/tools/getprivpw.asp. Paste in your entire cookie and it will spit out the rommon priv password.

It would also be wise to save the cookie as is right now in a text file. The cookie also contains the serial number encase you want to go back or incase you mess something up

Now that we have the password type priv and enter in the password. Now if you do question mark you will see a a lot more rommon commands.

rommon 2 > priv
Password:
You now have access to the full set of monitor commands.
Warning: some commands will allow you to destroy your
configuration and/or system images and could render
the machine unbootable.
rommon 3 >

To set the cookie we first need to find the hex vaules of your new desired serial number. There serial number is split into the first 3 digits being ASCII, next three are straight hex and the last four are ASCII digits again. For my example i will use RSCC1E19406 which will fit the bill. After we convert ASCII portions into hex we get 52:53:43:C1:E1:31:39:34:30:36. This needs to go into cookie at bytes 0x10 - 0x18. Keep the rest of the values the same! J

Type cookie again to modify the cookie. Hint: Just press enter on the rows you don't need to modify.

rommon 3 > cookie

View/alter bytes of serial cookie by field --
Input hex byte(s) or: CR -> skip field; ? -> list values
bytes 0x00-0x07: 01 01 00 09 43 ab 25 60
               >

bytes 0x08-0x0F: 43 20 00 ff 00 91 00 20
               >

bytes 0x10-0x17: 00 00 00 00 00 00 00 00
               >

bytes 0x18-0x1F: 52 53 43 cc 1e 31 39 34
               > 52 53 43 c1 e1 39 34 30

bytes 0x20-0x27: 30 36 00 00 00 00 00 00
               > 36 00 00 00 00 00 00 00

bytes 0x28-0x2F: 00 ff ff ff 50 06 49 0b
               >

bytes 0x30-0x37: 17 0f ff ff ff ff ff ff
               >

bytes 0x38-0x3F: ff ff ff ff ff ff ff ff
               >

bytes 0x40-0x47: ff ff ff ff ff ff ff ff
               >

bytes 0x48-0x4F: ff ff ff ff ff ff ff ff
               >

bytes 0x50-0x57: ff ff ff ff ff ff ff ff
               >

bytes 0x58-0x5F: ff ff ff ff ff ff ff ff
               >

bytes 0x60-0x67: ff ff ff ff ff ff ff ff
               >

bytes 0x68-0x6F: ff ff ff ff ff ff ff ff
               >

bytes 0x70-0x77: ff ff ff ff ff ff ff ff
               >

bytes 0x78-0x7F: ff ff ff ff ff ff ff ff
               >

Now just type boot to boot normal and do a show version to verify the results

Router>show ver | i board ID
Processor board ID RSCC1E19406 (4250487947)