3Com 4500G Family Configuration Manual page 849

DLDP timer
Enhanced timer
DelayDown timer
RecoverProbe timer
DLDP mode
DLDP can operate in two modes: normal mode and enhanced mode, as described below.
In normal DLDP mode, when an entry timer expires, the device removes the corresponding
neighbor entry and sends an Advertisement packet with the RSY tag.
In enhanced DLDP mode, when an entry timer expires, the Enhanced timer is triggered and the
device sends up to eight Probe packets at the frequency of one packet per second to test the
neighbor. If no Echo packet is received from the neighbor when the Echo timer expires, the device
transits to the Disable state.
Table 1-3 DLDP mode and neighbor entry aging
Detecting a neighbor
DLDP mode after the corresponding
neighbor entry ages out
Normal
No
DLDP mode
Enhanced
Yes
DLDP mode
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
state cannot be detected by common data link protocols, so the port on Device A is still up. In enhanced
DLDP mode, however, the port on Device A tests the peer port on Device B after the Entry timer
concerning the port on Device B expires. The port on Device A then transits to the Disable state if it
receives no Echo packet from the port on Device B when the Echo timer expires. As the port on Device
B is physically down, it is in the Inactive DLDP state.
In the enhanced mode, this timer is triggered if no packet is received
from a neighbor when the entry aging timer expires. Enhanced timer is
set to 1 second.
After the Enhanced timer is triggered, the device sends up to eight
probe packets to the neighbor at a frequency of one packet per second.
A device in the Active, Advertisement, or Probe DLDP link state transits
to DelayDown state rather than removes the corresponding neighbor
entry and transits to the Inactive state when it detects a port-down
event.
When a device transits to this state, the DelayDown timer is triggered. A
device in DelayDown state only responds to port-up events.
A device in the DelayDown state resumes its original DLDP state if it
detects a port-up event before the DelayDown timer expires. Otherwise,
it removes the corresponding DLDP neighbor information and transits to
the Inactive state.
This timer is set to 2 seconds. That is, a port in the Disable state sends
one RecoverProbe packet every two seconds to detect whether a
unidirectional link has restored.
Removing the neighbor
entry immediately after
the Entry timer expires
Yes
No
Figure 1-2 may occur, where the port on Device B is actually down but its
1-4
Description
Triggering the Enhanced
timer after an Entry timer
No
Yes
expires