You want to disable the banner on a particular port to prevent it from confusing an attached device such as a modem.
To disable banners on particular lines, use the following commands:
Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#line aux 0 Router1(config-line)#no motd-banner Router1(config-line)#no exec-banner Router1(config-line)#exit Router1(config)#end Router1#
By default, the router will display the configured banner messages on all of its lines. However, there are circumstances when these banner messages can confuse directly attached devices. For instance, modems connected to terminal servers or AUX lines can react erratically to banner messages. In these situations, you will need to disable the banner on the associated line.
Notice that you cannot disable the login banner on a line-by-line basis. So if you find that you need to do this, you should use the motd banner instead of the login banner to display the warning message. The motd and the EXEC banners are nearly identical, so this shouldn't cause any problems.