3com 5500-ei pwr Installation Instruction

 

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Table 1-4 DLDP operating mode and neighbor entry aging 

Normal 
mode 

No Yes 

No 

Enhanced 
mode 

Yes No 

Yes (When the enhanced timer 
expires, the state of the local end 
is set to unidirectional link, and 
the neighbor entry is aged out.) 

 

The enhanced DLDP mode is designed for addressing black holes. It prevents the cases where one end 

of a link is up and the other is down. If you configure the speed and the duplex mode by force on a 

device, the situation shown in 

 may occur, where Port B is actually down but the state of Port 

B cannot be detected by common data link protocols, so Port A is still up. In enhanced DLDP mode, 

however, Port A tests Port B after the Entry timer concerning Port B expires. Port A then transits to the 

Disable state if it receives no Echo packet from Port A when the Echo timer expires. As Port B is 

physically down, it is in the Inactive DLDP state. 

Figure 1-3 A case for Enhanced DLDP mode 

 

 

z 

In normal DLDP mode, only fiber cross-connected unidirectional links (as shown in 

be detected. 

z 

In enhanced DLDP mode, two types of unidirectional links can be detected. One is fiber 

cross-connected links (as shown in 

). The other refers to fiber pairs with one fiber not 

connected or disconnected (as shown in 

). To detect unidirectional links that are of the 

latter type, you need to configure the ports to operate at specific speed and in full duplex mode. 

Otherwise, DLDP cannot take effect. 

 

1)  If the DLDP-enabled link is up, DLDP sends DLDP packets to the peer device, and 

analyzes/processes the DLDP packets received from the peer device. DLDP packets sent in 

different DLDP states are of different types.