Two AR28 routers are interconnected through HDLC link. One protocol state is up and the other protocol state is down.
Delete no keepalive configureation and protocol state of both ends is down.
Loop and confirm that the interface is not problematic. Check transmission link.
HDLC parameters configuration of two routers are inconsistent and transmission link causes the problem. One end configures no keepalive, but the other end does not configure it.
HDLC can realize timing detection of the link by sending Keepalive information mutually. In Quidway router, Keepalive is enabled by default and the router interface periodically sends Keepalive packet. If response packet of the peer is not received after sending five consecutive Keepalive packets, HDLC state is down.
Furthermore, in Quidway router, if one port encapsulates HDLC protocol, close timing detection of the link through no keepalive command. Under such situation as long as the physical layer is up, the router will display that the link layer protocol is up and the port will not send keepalive packet externally. HDLC protocol of link layer will not be down because keepalive information of the peer is not received.