37
Figure 9
Correct and incorrect fiber connections
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
DLDP link states
State Indicates…
Initial
DLDP is disabled.
Inactive
DLDP is enabled, and the link is down.
Active
DLDP is enabled and the link is up, or the neighbor entries have been cleared.
