Overview of DLDP

Definition

The Device Link Detection Protocol (DLDP) monitors the link status of optical fibers or copper twisted pairs such as super Category 5 twisted pairs. Upon detecting a unidirectional link on an interface, DLDP automatically shuts down or prompts users to manually shut down the interface to prevent network faults.

Purpose

If a unidirectional link occurs in a network, a local device is able to receive packets from the remote device at the data link layer, but the remote device cannot receive packets from the local device. Unidirectional links result in problems such as loops on a spanning tree protocol (STP) topology.

In Figure 1 and Figure 2, as a result of intersected and disconnected fibers, a unidirectional link is detectable on the network.

Figure 1 Intersected fibers

Figure 2 Disconnected fibers

As a link layer protocol, DLDP works with physical layer protocols to detect the link status. The auto negotiation mechanism at the physical layer detects physical signals and faults, and DLDP identifies the remote device, detects unidirectional links, and shuts down unreachable interfaces. The auto negotiation mechanism and DLDP work together to detect and disable unidirectional links at physical and logical layers. If the interfaces on both ends of a link work properly at the physical layer, DLDP checks connections and packet exchange between the two interfaces at the link layer. This detection cannot be implemented using the auto negotiation mechanism.