1-2Figure 1-2 Fiber broken or not connectedDevice AGE1/0/49 GE1/0/50Device BGE1/0/49 GE1/0/50PCDevice link detection protocol (DLDP) can detect the link status of an optical fiber cable or coppertwisted pair (such as super category 5 twisted pair). If DLDP finds a unidirectional link, it disables therelated port automatically or prompts you to disable it manually according to the configurations, to avoidnetwork problems.A copper twisted-pair cable (such as a Category 5e twisted-pair cable) contains eight wires. Some ofthese wires only transmit data, while the others only receive data. When the wires that only receive dataor those that only transmit data all fail while the others are normal, a unidirectional link occurs.DLDP provides the following features:z As a link layer protocol, it works together with the physical layer protocols to monitor the link statusof a device.z The auto-negotiation mechanism at the physical layer detects physical signals and faults. DLDPidentifies peer devices and unidirectional links, and disables unreachable ports.z Even if both ends of links can work normally at the physical layer, DLDP can detect whether theselinks are connected correctly and whether packets can be exchanged normally at both ends.However, the auto-negotiation mechanism cannot implement this detection.DLDP FundamentalsDLDP packetsDLDP detects link status by exchanging the following types of packets.Table 1-1 DLDP packet typesDLDP packet type FunctionAdvertisementNotifies the neighbor devices of the existence of the local device. Anadvertisement packet carries only the local port information, and it doesnot require response from the peer end.
