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thisco
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Changing baud rate without access?

Thu Feb 18, 2010 5:38 pm

Hi guys,

Something of a pickle here - the baud rate on one of the switches has been set to a level that's too high for us to use (current com ports simply don't support it) - and was wondering if there was a way to reset the baud rate, or reduce it down whilst being presented with the odd characters, i.e. @@@@@@? I [believe] it was set to 921600, and our com ports don't support anywhere near that

I guess the worst case scenario is to buy a USB/Serial interface that supports the baud, but I was wondering if there was another way.

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Re: Changing baud rate without access?

Thu Feb 18, 2010 7:23 pm

I believe you can only get as high as 115200 for the speed of a serial connection via a com port to a router/switch. I might be wrong, but I have only ever seen speeds at that or below. There are only like 6-8 different speeds that a router switch will connect at.

Do a google search for the supported speeds and for setting the config register if you can get back into the switch.
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thisco
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Re: Changing baud rate without access?

Thu Feb 18, 2010 7:38 pm

wirerat wrote:I believe you can only get as high as 115200 for the speed of a serial connection via a com port to a router/switch. I might be wrong, but I have only ever seen speeds at that or below. There are only like 6-8 different speeds that a router switch will connect at.

Do a google search for the supported speeds and for setting the config register if you can get back into the switch.



OK, but if the switch was set to a speed higher than that in rommon, what is the solution? Is it possible to set a baud rate to a higher rate than is actually supported?! The issue is that the switch has no IOS, and being that none of the baud rates supported by our com ports are returning any values other than @@@@ and other odd characters - how can we change it back?

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Re: Changing baud rate without access?

Thu Feb 18, 2010 8:04 pm

What kind of switch is it? Cisco devices don't support a baud rate higher than 115200...

And how do you know the baud is set "too high"? There are other reasons you might see that on the console that are not related to baud.

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Re: Changing baud rate without access?

Thu Feb 18, 2010 8:19 pm

Infinite wrote:What kind of switch is it? Cisco devices don't support a baud rate higher than 115200...

And how do you know the baud is set "too high"? There are other reasons you might see that on the console that are not related to baud.


Catalyst 2960.

Well, I suppose that I don't know for sure - I can only go off what I was told before I had to head off. I only had a few minutes to spend on it, so I didn't cycle through all of the baud rates on the terminal emulator, but I was told that in rommon - he had set the baud to 921600 (again, he has told me 900,000-something, so I can only assume 921600). I'll have to look at it again on Monday - but is it possible in rommon to use 'set baud rate XXX' to be of an arbitrary value that the device cannot actually use?

It's too high because on the lower baud ranges I tested we got no output at all, but as we approached the higher ranges characters started to come through. When I select 921600 as the setting for our com port, it returns an error that it's not supported.

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Re: Changing baud rate without access?

Thu Feb 18, 2010 8:29 pm

Well, the supported console rates re determined by the bootloader. I don't suppose anything useful is displayed on the screen while it boots? (like the bootloader version)

I found this at http://www.cisco.com/en/US/docs/switche ... #wp1004653

BAUD rate—The rate in bits per second (bps) used for the console. The Cisco IOS software inherits the baud rate setting from the boot loader and continues to use this value unless the configuration file specifies another setting. The range is from 0 to 4294967295 bps. Valid values are 50, 75, 110, 150, 300, 600, 1200, 1800, 2000, 2400, 3600, 4800, 7200, 9600, 14400, 19200, 28800, 38400, 56000, 57600, 115200, and 128000.

The most commonly used values are 300, 1200, 2400, 9600, 19200, 57600, and 115200.


You also say:

I only had a few minutes to spend on it, so I didn't cycle through all of the baud rates on the terminal emulator


Go back and try them all. One by one. And be patient. It sucks, I know. I've done this before my self. I've even gone through them all once with no success and tried again only to discover one I've tried once worked on the second attempt.

Whoever told you they set it to 900,000 something is full of crap. What is more likely is they don't know what they set it to and are just spouting random numbers at you. Tell them they're an idiot and go show them how to recover it ;)

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Re: Changing baud rate without access?

Thu Feb 18, 2010 8:31 pm

i would go back and check all the common ones 1200,2400,4800,9600 . Console ports do not go above 115200. Make sure you are using a good emulator like Tera Term .

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Re: Changing baud rate without access?

Thu Feb 18, 2010 8:33 pm

There are only 3 bits in the config register used for console baud speed. Here's a table of them all:

https://www.cisco.com/en/US/docs/router ... ml#wp40327

900,000 something is not an option ;)

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Re: Changing baud rate without access?

Thu Feb 18, 2010 9:00 pm

Thanks guys. Hopefully it is just a case of it being one of the rates I didn't check.

I do wonder though, what's this about the range?
The range is from 0 to 4294967295 bps
. I suppose that my fear is that when in rommon there are no checks to see if the input value is actually valid or not, and perhaps accepted any value?

If there are only 8 or so values that are valid, why is the range so long? Forgive my ignorance, but I've not played too much with baud rates myself.

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Re: Changing baud rate without access?

Thu Feb 18, 2010 9:46 pm

I thought that was weird when I read it too, but look at the complete sentence.

The range is from 0 to 4294967295 bps. Valid values are 50, 75, 110, 150, 300, 600, 1200, 1800, 2000, 2400, 3600, 4800, 7200, 9600, 14400, 19200, 28800, 38400, 56000, 57600, 115200, and 128000.

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Re: Changing baud rate without access?

Fri Feb 19, 2010 5:54 am

Just another thought, are you using the other required values for connection to a console port?

8 data bits
no parity
1 stop bit
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thisco
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Re: Changing baud rate without access?

Sat Feb 20, 2010 3:46 am

wirerat wrote:Just another thought, are you using the other required values for connection to a console port?

8 data bits
no parity
1 stop bit



Yes, all the default settings were used. I'll be able to find out come Monday, but it's still bugging me a lot right now.

Does anyone have access to a switch, or virtual switch that can use rommon? I would really like to see what value it sets to if manually changed in rommon. For example - set baud rate ....

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Re: Changing baud rate without access?

Sat Feb 20, 2010 6:51 am

Does anyone have access to a switch, or virtual switch that can use rommon? I would really like to see what value it sets to if manually changed in rommon. For example - set baud rate ....


Have a look at the links below. They should tell you all you need to know to about the baud rate and rommon mode. In my experience with switch rommon modes and changing the baud rate it gives you a menu to select one of the values that are shown in the links below. There was no way to put in a different baud rate than what choices they gave you.

http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a008022493f.shtml
https://supportforums.cisco.com/docs/DOC-4517;jsessionid=549751E8980D6468054E1D0AD93DE124.node0
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Re: Changing baud rate without access?

Sun Feb 10, 2019 2:56 pm

i also have issue with baud rates

I bought a cisco 2951 service router and a 48 port service module to practice on for qualification,



i setup the 2951, and managed to connect to the 48 port switch module using the following command,



service-module GigabitEthernet2/0 session



i stupidly set the baud to 57600 within the service module etherswitch and now i can access but with strange chars,



however i can run a status on it



midnight-cisco2951#service-module GigabitEthernet2/0 status

Service Module is Cisco GigabitEthernet2/0

Service Module supports session via TTY line 131

Service Module is trying to recover from error

Service Module reset on error is disabled

Service Module heartbeat-reset is enabled

Service Module status is not available



i tried the following



midnight-cisco2951(config-line)#speed 57600

Only baud rates 9600 and 115200 are supported

Failed to change line 131's speed

midnight-cisco2951(config-line)#



this apparently supposed to change the baud rate on the connection, but it would not allow 57600.



i have set the baud on the 2951 router rommon to 57600 thinking this might work, but did not.



how do i go about clearing this



Dave

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