3Com 7757 Configuration Manual page 213

Table 142 DLDP packet types
DLDP packet type
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RSY-Advertisement
packets (referred to as
RSY packets hereafter)
Flush-Advertisement
packets (referred to as
flush packets hereafter)
Probe
Echo
Disable
LinkDown
Recover Probe
Function
Notifies the neighbor devices of the existence of the local device. An
advertisement packet carries only the local port information, and it
does not require response from the peer end.
Advertisement packet with the RSY flag set to 1. RSY advertisement
packets are sent to request synchronizing the neighbor information
when neighbor information is not locally available or a neighbor
information entry ages out.
Advertisement packet with the flush flag set to 1. A flush packet
carries only the local port information (instead of the neighbor
information) and is used to trigger neighbors to remove the
information about the local device.
Probe packets are used to probe the existence of a neighbor. Echo
packets are required from the corresponding neighbor. Probe
packets carry the local port information. Neighbor information is
optional for probe packets. A probe packet carrying neighbor
information probes the specified neighbors; A probe packet carrying
no neighbor information probes all the neighbors.
Response to probe packets. An echo packet carries the information
about the response port and the neighbor information it maintains.
Upon receiving an echo packet, a port checks whether the neighbor
information carried in the echo packet is consistent with that of
itself. If yes, the link between the local port and the neighbor is
regarded as bidirectional.
Disable packets are used to notify the peer end that the local end is
in the disable state. Disable packets carry only the local port
information instead of the neighbor information. When a port
detects a unidirectional link and enters the disable state, the port
sends disable packets to the neighbor. A port enters the disable
state upon receiving a disable packet.
Linkdown packets are used to notify unidirectional link emergencies
(a unidirectional link emergency occurs when the local port is down
and the peer port is up). Linkdown packets carry only the local port
information instead of the neighbor information. In some
conditions, a port is considered to be physically down if the link
connecting to the port is physically abnormal (for example, the Rx
line of the fiber on the port is disconnected, 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 still considered to be
normal. Such a situation is known as unidirectional link emergency.
When a unidirectional link emergency occurs, DLDP sends linkdown
packets immediately to inform the peer of the link abnormality.
Without linkdown packets, the peer can detect the link abnormality
only after a period when the corresponding neighbor information
maintained on the neighbor device ages out, which is three times
the advertisement interval. Upon receiving a linkdown packet, if the
peer end operates in the enhanced mode, it enters the disable state,
and sets the receiving port to the DLDP down state (auto shutdown
mode) or gives an alarm to the user (manual shutdown mode).
Recover probe packets are used to detect whether a link recovers to
implement the port auto-recovery mechanism. Recover probe
packets carry only the local port information instead of the neighbor
information. They request for recover echo packets as the response.
A port in the DLDP down state sends a recover probe packet every
two seconds.
DLDP Fundamentals 213