Demystifying Cisco Config Register Bits

Tony Mattke · 2009.06.08 · 2 min read

Ever accidentally set your config register to a random value that isn’t in the Cisco documentation? No? Neither have I, but one day I encountered someone on #cisco that had. So I did my best to help him, and was able to decode his settings and get him back into the CLI.

First things first. Lets hope you remember the value you entered, or else you’re going to be trying every bit rate.. If you do, convert it to decimal then binary, or straight to binary if you’re a badass. For example lets use 0x2121. Cisco has no reference of this in their documentation. Now that you can’t get into your router at the typical 9600 8,N,1…

Convert 0x2121 to decimal………….. 8481
Convert 8481 to binary…………………. 10000100100001
Insert this number into the table below… Top->Bottom; Right->Left
I’ve filled in the values on the far left column.

Bit	Feature	0x2121	9600	19200	4800	38400	1200	57600	2400	115200
0	Boots field parameters	1	0	0	0	0	0	0	0	0
1	same as above	0	1	0	1	1	1	1	1	1
2	same as above	0	0	0	0	0	0	0	0	0
3	same as above	0	0	0	0	0	0	0	0	0
4	????????	0	0	0	0	0	0	0	0	0
5	console line speed	1	0	1	0	1	0	1	0	1
6	ignore NVRAM	0	0	0	0	0	0	0	0	0
7	disable boot messages	0	0	0	0	0	0	0	0	0
8	break disabled	1	1	1	1	1	0	1	1	1
9	?????????	0	0	0	0	0	1	0	0	0
10	ip broadcast with all zeros	0	0	0	0	0	0	0	0	0
11	console line speed	0	0	0	1	1	0	0	1	1
12	console line speed	0	0	0	0	0	1	1	1	1
13	Boots default rom software if network boot fails	1	0	1	1	1	0	1	1	1
14	IP broadcasts do not have net numbers	0	0	0	0	0	0	0	0	0
15	Enables Diagnostic Messages	0	0	0	0	0	0	0	0	0