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The Device link detection protocol (DLDP) detects unidirectional links (fiber links or twisted-pair links) and
can be configured to shut down the related port automatically or prompt users to take actions to avoid
network problems.
As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor link status. When
the 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 shutting down
unreachable ports. The auto-negotiation mechanism and DLDP work together to ensure that
physical/logical unidirectional links can be detected and shut down, and to prevent failure of other
protocols such as STP. If both ends of a link are operating normally at the physical layer, DLDP detects
whether the link is correctly connected at the link layer and whether the two ends can exchange packets
properly. This is beyond the capability of the auto-negotiation mechanism at the physical layer.
How DLDP works
DLDP link states
A switch is in one of these DLDP link states: Initial, Inactive, Active, Advertisement, Probe, Disable, and
DelayDown, as described in Table 10.
Table 10 DLDP link states
DLDP is enabled, and the link is down.
DLDP is enabled and the link is up, or the neighbor entries have been cleared.
All neighbors are bi-directionally reachable or DLDP has been in active state for
more than five seconds. This is a relatively stable state where no unidirectional
link has been detected.
DLDP enters this state if it receives a packet from an unknown neighbor. In this
state, DLDP sends packets to check whether the link is unidirectional. As soon as
DLDP transits to this state, a probe timer starts and an echo timeout timer starts for
each neighbor to be probed.
A port enters this state when:
A unidirectional link is detected.
The contact with the neighbor in enhanced mode gets lost.
In this state, the port does not receive or send packets other than DLDPDUs.
A port in the Active, Advertisement, or Probe DLDP link state transits to this state
rather than removes the corresponding neighbor entry and transits to the Inactive
state when it detects a port-down event. When a port transits to this state, the
DelayDown timer is triggered.
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