1-2
Figure 1-2 Fiber broken or not connected
Device A
GE1/0/49 GE1/0/50
Device B
GE1/0/49 GE1/0/50
PC
Device link detection protocol (DLDP) can detect the link status of an optical fiber cable or copper
twisted pair (such as super category 5 twisted pair). If DLDP finds a unidirectional link, it disables the
related port automatically or prompts you to disable it manually according to the configurations, to avoid
network problems.
A copper twisted-pair cable (such as a Category 5e twisted-pair cable) contains eight wires. Some of
these wires only transmit data, while the others only receive data. When the wires that only receive data
or those that only transmit data all fail while the others are normal, a unidirectional link occurs.
DLDP provides the following features:
z As a link layer protocol, it works together with the physical layer protocols to monitor the link status
of a device.
z The auto-negotiation mechanism at the physical layer detects physical signals and faults. DLDP
identifies peer devices and unidirectional links, and disables unreachable ports.
z Even if both ends of links can work normally at the physical layer, DLDP can detect whether these
links are connected correctly and whether packets can be exchanged normally at both ends.
However, the auto-negotiation mechanism cannot implement this detection.
DLDP Fundamentals DLDP packets
DLDP detects link status by exchanging the following types of packets.
Table 1-1 DLDP packet types
DLDP packet type Function
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Notifies the neighbor devices of the existence of the local device. An
advertisement packet carries only the local port information, and it does
not require response from the peer end.