1-2Figure 1-1 Correct and incorrect fiber connectionsCorrect fiber conecton Unidirectional connection type 1Cross-connected fibersUnidirectional connection type 2One fiber of a fiber pair Is notconnected or Is brokenGE1/0/50 GE1/0/51Device ADevice BGE1/0/50Device ADevice BGE1/0/50Device ADevice BEthernetoptical port Unconnected or broken fiberFiber linkGE1/0/51 GE1/0/51GE1/0/50 GE1/0/51 GE1/0/50 GE1/0/51 GE1/0/50 GE1/0/51Tx end Rx endThe Device link detection protocol (DLDP) is an technology for dealing with unidirectional links that mayoccur in a network. On detecting a unidirectional link, DLDP, as configured, can shut down the relatedport automatically or prompt users to take actions to avoid network problems.As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor link status. Whilethe auto-negotiation mechanism provided by the physical layer detects physical signals and faults,DLDP performs operations such as identifying peer devices, detecting unidirectional links, and shuttingdown unreachable ports. The cooperation of physical layer protocols and DLDP ensures thatphysical/logical unidirectional links can be detected and shut down and prevents failure of otherprotocols such as STP. If both ends of a link are operating normally at the physical layer, DLDP detectswhether the link is correctly connected at the link layer and whether the two ends can exchange packetsproperly. This is beyond the capability of the auto-negotiation mechanism at the physical layer.How DLDP WorksDLDP link statesA device is in one of these DLDP link states: Initial, Inactive, Active, Advertisement, Probe, Disable, andDelayDown, as described in Table 1-1.Table 1-1 DLDP link statesState Indicates…Initial DLDP is disabled.Inactive DLDP is enabled but the link is down.Active DLDP is enabled and the link is up, or the neighbor entries havebeen cleared.
