Configuration guidelines
You can enable SLIP encapsulation only on asynchronous interfaces. For a synchronous/asynchronous interface, you can switch it to the asynchronous mode and then enable SLIP encapsulation on it.
Some asynchronous interfaces do not support the SLIP protocol, but you can still configure the link-protocol slip command on it. When you do that, the system informs you that the operation of SLIP encapsulation fails. By then, no encapsulation is enabled on the interface. You must use the link-protocol ppp command to enable PPP encapsulation on the interface to make it available again.