3Com Switch 4800G 24-Port Configuration Manual page 187

Table 32 DLDP mode and neighbor entry aging
DLDP mode
Normal DLDP mode
Enhanced DLDP mode Yes
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 51
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 51 A case for Enhanced DLDP mode
Tx
Port A
Rx
up
n
In normal DLDP mode, only fiber cross-connected unidirectional links (as shown
■
in Figure 49) can be detected.
In enhanced DLDP mode, two types of unidirectional links can be detected.
■
One is fiber cross-connected links (as shown in Figure 49). The other refers to
fiber pairs with one fiber not connected or disconnected (as shown in
Figure 50). 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. When a fiber of a fiber pair is not
connected or gets disconnected, the port that can receive optical signals is in
Disable state; the other port is in Inactive state.
DLDP authentication mode
You can prevent network attacks and illegal detect through DLDP authentication.
Three DLDP authentication modes exist, as described below.
Non-authentication. In this mode, the sending side sets the Authentication
■
field and the Authentication type field of DLDP packets to 0. The receiving side
checks the values of the two fields of received DLDP packets and drops the
packets with the two fields conflicting with the corresponding local
configuration.
Plain text authentication. In this mode, before sending a DLDP packet, the
■
sending side sets the Authentication field to the password configured in plain
text and sets the Authentication type field to 1. The receiving side checks the
values of the two fields of received DLDP packets and drops the packets with
the two fields conflicting with the corresponding local configuration.
Detecting a neighbor Removing the
after the neighbor entry
corresponding immediately after
neighbor entry ages the Entry timer
out expires
No Yes
No
Rx
Port B
Tx
down
Overview 187
Triggering the
Enhanced timer
after an Entry
timer expires
No
Yes