3com 5500-ei pwr Installation Instruction

 

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 detects link status by exchanging the following types of packets.  

Table 1-1 DLDP packet types 

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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.