1-2
Figure 1-2 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
DLDP Introduction
Device Link Detection Protocol (DLDP) can detect the link status of a fiber cable or twisted pair. 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 device. 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 be detected and shut down. For a link with the devices on the
both sides of it operating properly, DLDP checks to see if the cable is connected correctly and if packets
can be exchanged between the two devices. Note that DLDP is not implemented through
auto-negotiation.
DLDP Fundamentals
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 1-1.
Table 1-1 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.
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