In the establishment of negotiation of PPP LCP, if the LCP response packet from the peer is not received within 3 seconds, the previous LCP config_request packet will be transmitted again.
If PPP negotiation succeeds, and link is established, PPP makes sure the peer exists via the polling packet of echo request/echo reply.
PPP negotiation interval:command: ppp timer negotiate "1-10", with second as unit; timeout time is defaulted to 3 seconds.
PPP polling interval: command: timer hold "0~32767", with second as unit. By default, the timeout interval is 10 seconds. When it is set to 0, it does not verify the validity of link.