1-2Figure 1-2 Unidirectional fiber link: a fiber not connected or disconnectedDevice AGE1/0/49 GE1/0/50Device BGE1/0/49 GE1/0/50PCDLDP IntroductionThe 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.DLDP FundamentalsDLDP 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.AdvertisementAll neighbors are bi-directionally reachable or DLDP has been inactive state for more than five seconds. This is a relatively statewhere no unidirectional link has been detected.
