1-3DLDP packet type FunctionRSY-Advertisementpackets (referred to asRSY packets hereafter)Advertisement packet with the RSY flag set to 1. RSY advertisementpackets are sent to request synchronizing the neighbor information whenneighbor information is not locally available or a neighbor informationentry ages out.Flush-Advertisementpackets (referred to asflush packets hereafter)Advertisement packet with the flush flag set to 1. A flush packet carriesonly the local port information (instead of the neighbor information) and isused to trigger neighbors to remove the information about the localdevice.ProbeProbe packets are used to probe the existence of a neighbor. Echopackets are required from the corresponding neighbor. Probe packetscarry the local port information. Neighbor information is optional for probepackets. A probe packet carrying neighbor information probes thespecified neighbors; A probe packet carrying no neighbor informationprobes all the neighbors.EchoResponse to probe packets. An echo packet carries the information aboutthe response port and the neighbor information it maintains. Uponreceiving an echo packet, a port checks whether the neighbor informationcarried in the echo packet is consistent with that of itself. If yes, the linkbetween the local port and the neighbor is regarded as bidirectional.DisableDisable packets are used to notify the peer end that the local end is in thedisable state. Disable packets carry only the local port information insteadof the neighbor information. When a port detects a unidirectional link andenters the disable state, the port sends disable packets to the neighbor. Aport enters the disable state upon receiving a disable packet.LinkDownLinkdown packets are used to notify unidirectional link emergencies (aunidirectional link emergency occurs when the local port is down and thepeer port is up). Linkdown packets carry only the local port informationinstead of the neighbor information. In some conditions, a port isconsidered to be physically down if the link connecting to the port isphysically abnormal (for example, the Rx line of the fiber on the port isdisconnected, while the Tx line operates properly). But for the peer end,as Rx signals can still be received on the physical layer, the port is stillconsidered to be normal. Such a situation is known as unidirectional linkemergency.When a unidirectional link emergency occurs, DLDP sends linkdownpackets immediately to inform the peer of the link abnormality. Withoutlinkdown packets, the peer can detect the link abnormality only after aperiod when the corresponding neighbor information maintained on theneighbor device ages out, which is three times the advertisement interval.Upon receiving a linkdown packet, if the peer end operates in theenhanced mode, it enters the disable state, and sets the receiving port tothe DLDP down state (auto shutdown mode) or gives an alarm to the user(manual shutdown mode).Recover ProbeRecover probe packets are used to detect whether a link recovers toimplement the port auto-recovery mechanism. Recover probe packetscarry only the local port information instead of the neighbor information.They request for recover echo packets as the response. A port in theDLDP down state sends a recover probe packet every two seconds.Recover EchoRecover echo packets are response to recover probe packets in the portauto-recovery mechanism. A link is considered to restore to thebidirectional state if a port on one end sends a recover probe packet,receives a recover echo packet, and the neighbor information containedin the recover echo packet is consistent with that of the local port.
