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Table 1-4 DLDP operating mode and neighbor entry aging
DLDP
operating
mode
Detecting a neighbor
after the corresponding
neighbor entry ages
out
Removing the
neighbor entry
immediately after the
Entry timer expires
Triggering the Enhanced timer
after an Entry timer expires
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
Figure 1-3 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 Figure 1-1 ) can
be detected.
z In enhanced DLDP mode, two types of unidirectional links can be detected. One is fiber
cross-connected links (as shown in
Figure 1-1). The other refers to fiber pairs with one fiber not
connected or disconnected (as shown in
Figure 1-2). 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.
DLDP Implementation
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.