If 2 routers that are connected by a serial link are running 2 different encapsulations (for example, the default HDLC on one and PPP on the other) they will not communicate. I know you can see what encapsulation is on your router with the SHOW INTERFACE command, but how can you find the encapsulation type on the other router? I think it involves a DEBUG command, but I don't know which one to turn on.

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You could telnet into it if they were communicating. You could dial into it if there is a modem on the aux or con port.

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Well, if you have the protocol up and both routers communicating, I dont see a reason for debugging. If there is a problem in the link protocol and you are suspecting the encapsulation used on the otherside, you can use,

debug serial packet
or
debug serial interface

If you have more than one interface active on the router, you may need to configure an access-list and then use the command,

debug list serial <serial interface #> [<access-list #>]

Let me know if this works for you...

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