H3C Technologies H3C S3100 Series Switches - Manual (Page 195)

1-6
Timer Description
DelayDown timer
When a device in the active, advertisement, or probe DLDP state
receives a port down message, it does not removes the
corresponding neighbor immediately, neither does it changes to the
inactive state. Instead, it changes to the delaydown state first.
When a device changes to the delaydown state, the related DLDP
neighbor information remains, and the DelayDown timer is triggered.
The DelayDown timer is configurable and ranges from 1 to 5 seconds.
A device in the delaydown state only responds to port up messages.
A device in the delaydown state resumes its original DLDP state if it
receives a port up message before the delaydown timer expires.
Otherwise, it removes the DLDP neighbor information and changes to
the inactive state.
DLDP Operating Mode
DLDP can operate in two modes: normal mode and enhanced mode, as described below.
z In normal DLDP mode, when an entry timer expires, the device removes the corresponding
neighbor entry and sends an Advertisement packet with RSY tag.
z In enhanced DLDP mode, when an entry timer expires, the Enhanced timer is triggered and the
device sends up to eight Probe packets at a 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 1-4 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.