1-6Timer DescriptionDelayDown timerWhen a device in the active, advertisement, or probe DLDP statereceives a port down message, it does not removes thecorresponding neighbor immediately, neither does it changes to theinactive state. Instead, it changes to the delaydown state first.When a device changes to the delaydown state, the related DLDPneighbor 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 itreceives a port up message before the delaydown timer expires.Otherwise, it removes the DLDP neighbor information and changes tothe inactive state.DLDP Operating ModeDLDP 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 correspondingneighbor 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 thedevice sends up to eight Probe packets at a frequency of one packet per second to test theneighbor. If no Echo packet is received from the neighbor when the Echo timer expires, the devicetransits to the Disable state.Table 1-4 DLDP operating mode and neighbor entry agingDLDPoperatingmodeDetecting a neighborafter the correspondingneighbor entry agesoutRemoving theneighbor entryimmediately after theEntry timer expiresTriggering the Enhanced timerafter an Entry timer expiresNormalmode No Yes NoEnhancedmode Yes NoYes (When the enhanced timerexpires, the state of the local endis set to unidirectional link, andthe neighbor entry is aged out.)The enhanced DLDP mode is designed for addressing black holes. It prevents the cases where one endof a link is up and the other is down. If you configure the speed and the duplex mode by force on adevice, the situation shown in Figure 1-3 may occur, where Port B is actually down but the state of PortB 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 theDisable state if it receives no Echo packet from Port A when the Echo timer expires. As Port B isphysically down, it is in the Inactive DLDP state.
