Trying to telnet to a 6500 switch I receive 3 user name prompts then the session closes out before I can login

[djs151515]

When trying to telnet to a 6500 switch I receive 3 user name prompts then the session closes out

before I can type in my login credentials. This just started happening on 2 switches yesterday. Any

help would be greatly  appreciated. Thanks 

[Peter Paluch]

Hello,

Do you have an option of trying different Telnet clients - say, the built-in telnet command in Windows, PuTTY, Tera Term, Linux-based telnet or perhaps a telnet command issued from another IOS-based switch? We need to make absolutely sure that this issue is caused by the switch you are telnetting into, and not by the Telnet client you're currently using.

Best regards,
Peter

[djs151515]

I have tried Windows Telnet , Putty and Terra Term with the same result. I have also tried to telnet from other switches and still same result. I am wondering if the switch has too many vty sessions

open ? 

[Peter Paluch]

Hi,

I have tried Windows Telnet , Putty and Terra Term with the same result.

I see. Hmmm. This sounds serious.

I am wondering if the switch has too many vty sessions open ? 

This wouldn't be a typical behavior although at this point, I am not going to exclude any possibility. In any case, the number of telnet sessions into a device can be checked using show users that will display all users that are logged into the device.

Are there any messages logged in the show logging output - perhaps mentioning issues with free memory allocation?

Are you also running SSH on your switches? Is the SSH affected as well?

Best regards,
Peter

[djs151515]

Unfortunately we do not have SSH configured. I am unable to access the switch via telnet as it closes out before I can log in to check the users. All the IPs on the switch are pingable and traffic is passing through it. I tried the ctrl-shift-6-x to try to break the session but no luck. Strange this happened to (2) switches in the last 2 days.

[djs151515]

Just adding the screen that appears before the session closes :

User Access Verification

Username:
Username:
Username:

[Peter Paluch]

Hello,

I see. Somehow, this looks either like a bug, or a memory exhaustion. Are you able to use the console port to access the switch? Alternatively, is the switch configured to send Syslog messages to a logging server? If so, it is important to check the logging messages generated by that device. I suspect that they will reveal some interesting information.

Best regards,
Peter

[djs151515]

I will try to console in to see whats going on. I did see this config line in a backed up config from the switch. Thanks for your help. 

line vty 0 4
 session-timeout 35791
 login local
 transport input lat pad mop udptn telnet rlogin ssh nasi acercon

[Peter Paluch]

Hello,

That line merely enumerates all remote management protocols that can be used to access the device. Most of the protocols in that line are not truly supported by your switch even though they are displayed. The only two ones that you're interested in are telnet and ssh (with SSH requiring further configuration to be actually usable).

The session-timeout is configured to an insanely large value, though, to obviously disable the automatic logging-out after a period of no activity. This is a questionable practice, and I strongly recommend against using it.

Best regards,
Peter

[djs151515]

Thanks for all your help. 

[djs151515]

After some research it turns out to be a Cisco bug that puts a "Control-C Typed" in the status of  the vty line 0. I tested this by logging in to the switch with 2 seperate telnet sessions simultaneously. I got in on line vty 1 and disconnected on vty 0. Now I am try to figure out how to only disable line vty 0 so we can use Lines 1-4 until we reboot. Listed below is the status of line vty 0. Thanks 

Status: Ready, No Exit Banner, Control-c Typed 

[Peter Paluch]

Hello,

Oh, I see. That's interesting.

Regarding the way of disabling VTY 0 - this should be possible by entering the following commands:

line vty 0
 transport input none

Best regards,
Peter

[djs151515]

Thanks again for all your help. After disabling vty 0 will the first telnet session now default to vty 1? I am afraid if I disable vty 0  I will not be able to telnet to the switch.

Thanks 

[Peter Paluch]

Hi,

Don't worry. Setting up vty 0 affects only VTY 0. Logins should continue to be served by remaining VTY lines, so the first telnet session should then go to VTY 1. I've tested this on a 12.4T router IOS and on a Catalyst 3560G running 12.2(46)SE.

Best regards,
Peter

[djs151515]

Ok great. Thanks again. You have been very helpful.