32Figure 4 Unidirectional fiber link: fiber either not connected, or disconnectedDevice AGE2/0/1 GE2/0/2Device BGE2/0/1 GE2/0/2PCDLDP can detect the link status of twisted pair or fiber cable. On detecting a unidirectional link,DLDP can shut down the related port automatically or prompt users to take measures asconfigured to avoid network problems.As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor the linkstatus of a switch. The auto-negotiation mechanism provided by physical layer protocols detectsphysical signals and faults. DLDP, however, performs operations such as identifying peerswitches, detecting unidirectional links, and shutting down unreachable ports. Cooperationbetween physical layer protocols and DLDP ensures that physical/logical unidirectional links aredetected and shut down. When the switches at both ends of a link are operating properly, DLDPchecks whether the cable is connected correctly and whether packets can be exchanged betweenthe two switches. This is beyond the capability of the auto-negotiation mechanism at the physicallayer.How DLDP worksDLDP link statesA switch is in one of the following DLDP link states: Initial, Inactive, Active, Advertisement, Probe,Disable, and DelayDown, as described in Table 9.Table 9 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 for morethan five seconds. This is a relative state where no unidirectional link has beendetected.
