4-2
Figure 4-2
Unidirectional fiber link: a fiber not connected or disconnected
Device A
Device B
PC
GE1/0/50
GE1/0/50
GE1/0/51
GE1/0/51
The Device Link Detection Protocol (DLDP) can detect the link status of a fiber cable or twisted pair.
On detecting a unidirectional link, DLDP, as configured, can 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.
While 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 cooperation of physical layer protocols and DLDP ensures that
physical/logical unidirectional links can be detected and shut down and prevents 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 device is in one of these DLDP link states: Initial, Inactive, Active, Advertisement, Probe, Disable,
and DelayDown, as described in
Table 4-1
.
Table 4-1
DLDP link states
State
Indicates…
Initial
DLDP is disabled.
Inactive
DLDP is enabled but the link is down.
Active
DLDP is enabled and the link is up, or the neighbor entries have
been cleared.
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All neighbors are bi-directionally reachable or DLDP has been in
active state for more than five seconds. This is a relatively state
where no unidirectional link has been detected.
Probe
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.
