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Catalyst switch running Integrated IOS may go into ROMmon mode when a terminal is connected to the console port due to the break key being enabled

Core issue

Configuration register is set to break enabled, resulting in the Multilayer Switch Feature Card (MSFC) going to ROMmon any time a break is issued.

Resolution

Plugging a terminal into the console port periodically produces    a ROMmon prompt. A misbehaving or reloading terminal may accidentally send the    router a signal similar to the break sequence. If the MSFC configuration    register is set to "break enable" this will put the MSFC in ROMmon mode.

To disable the break key, reset the router from ROMmon mode to boot the router    to normal router mode.

Issue a show version command and check the configuration register setting    on the last line of the output.   If the configuration register is set to one of the following, the break key    is enabled:

Configuration register is 0x2
or
Configuration register is 0x2002

Setting the configuration regitster to 0x2102 disables the break key. To change config register to 0x2102, issue the following command:

C6006-Y> (enable) switch console
Trying Router-15...
Connected to Router-15.
Type ^C^C^C to switch back...
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#config-reg 0x2102
Router(config)#^Z
Router#wr mem

The new configuration register setting will not take affect until the switch    has been reloaded. Configuration register is 0x2 (will be 0x2102 at next reload).

Switch Access

Console

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