1-2Figure 1-2 Unidirectional fiber link: a fiber not connected or disconnectedDevice ADevice BPCGE1/0/50GE1/0/50GE1/0/51GE1/0/51DLDP IntroductionDevice Link Detection Protocol (DLDP) can detect the link status of a fiber cable or twisted pair. Ondetecting a unidirectional link, DLDP can shut down the related port automatically or prompt users totake measures as configured to avoid network problems.As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor the link status ofa device. While the auto-negotiation mechanism provided by the physical layer detects physical signalsand faults, DLDP performs operations such as identifying peer devices, detecting unidirectional links,and shutting down unreachable ports. The cooperation of physical layer protocols and DLDP ensuresthat physical/logical unidirectional links be detected and shut down. For a link with the devices on theboth sides of it operating properly, DLDP checks to see if the cable is connected correctly and if packetscan be exchanged between the two devices. Note that DLDP is not implemented throughauto-negotiation.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 have been cleared.AdvertisementAll neighbors are bi-directionally reachable or DLDP has been in active state formore than five seconds. This is a relatively state where no unidirectional link hasbeen detected.ProbeDLDP enters this state if it receives a packet from an unknown neighbor. In thisstate, DLDP sends packets to check whether the link is unidirectional. As soonas DLDP transits to this state, a probe timer starts and an echo timeout timerstarts for each neighbor to be probed.
