Cisco HDLC (cHDLC) has two timers associated with the protocol, the keepalive interval and the timeout interval. The keepalive interval sends periodic keepalive packets. The receiver process expects to receive a keepalive packet at the rate specified by the keepalive interval. The link is declared down if the receiver process does not receive a keepalive within the timeout interval. The link is declared up when the number of continual keepalive packets received equals the up-count.
It is recommended that the nodes at the two endpoints of the cHDLC link are provisioned with the same values.