1-2Figure 1-2 Unidirectional fiber link: a fiber not connected or disconnectedDevice AGE2/0/1 GE2/0/2Device BGE2/0/1 GE2/0/2PCThe Device Link Detection Protocol (DLDP) can detect the link status of a fiber cable or twisted pair. Ondetecting a unidirectional link, DLDP, as configured, can shut down the related port automatically orprompt 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 have beencleared.AdvertisementAll neighbors are bi-directionally reachable or DLDP has been in activestate for more than five seconds. This is a relatively state where nounidirectional link has been detected.ProbeDLDP enters this state if it receives a packet from an unknown neighbor.In this state, DLDP sends packets to check whether the link isunidirectional. As soon as DLDP transits to this state, a probe timerstarts and an echo timeout timer starts for each neighbor to be probed.
