Overview
A link becomes unidirectional when only one end of the link can receive packets from the other end.
Unidirectional fiber links occur in the following cases:
Fibers are cross-connected.
A fiber is not connected at one end or one fiber of a fiber pair is broken.
Figure 6 shows a correct fiber connection and two types of unidirectional fiber connections.
Figure 6: Correct and incorrect fiber connections
Physical layer detection mechanisms, such as auto-negotiation, can detect physical signals and faults. However, they cannot detect communication failures for unidirectional links where the physical layer is in connected state.
As a data link layer protocol, the Device Link Detection Protocol (DLDP) detects the following:
Whether the fiber link or twisted-pair link is correctly connected at the link layer.
Whether the two ends of the link can exchange packets correctly.
When DLDP detects unidirectional links, it can automatically shut down the faulty port to avoid network problems. Alternatively, a user can manually shut down the faulty port. DLDP cooperates with physical layer protocols to monitor link status and avoid physical and logical unidirectional links.