How Dldp Works - H3C S9500E Series Configuration Manual

Figure 4 Unidirectional fiber link: fiber either not connected, or disconnected
Device A
GE2/0/1
GE2/0/1
Device B
DLDP 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 as
configured to avoid network problems.
As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor the link
status of a switch. The auto-negotiation mechanism provided by physical layer protocols detects
physical signals and faults. DLDP, however, performs operations such as identifying peer
switches, detecting unidirectional links, and shutting down unreachable ports. Cooperation
between physical layer protocols and DLDP ensures that physical/logical unidirectional links are
detected and shut down. When the switches at both ends of a link are operating properly, DLDP
checks whether the cable is connected correctly and whether packets can be exchanged between
the two switches. 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 the following DLDP link states: Initial, Inactive, Active, Advertisement, Probe,
Disable, and DelayDown, as described in Table 9.
Table 9 DLDP link states
State
Initial
Inactive
Active
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GE2/0/2
GE2/0/2
PC
Indicates...
DLDP is disabled.
DLDP is enabled but 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 relative state where no unidirectional link has been
detected.
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