Hello,
let's say you have a TELNET or SSH session from R1 to R2. In order to see log or debug messages from R2 on R1, through that TELNET or SSH session, you need to configure 'terminal monitor' on R1. So, basically, by default, these messages are not sent through the terminal session unless you have that command configured on the device that initiates the session.
Does that make sense ?
