H3C Technologies H3C S10500 Series Switches - Manual (Page 41)

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performs operations such as identifying peer devices, detecting unidirectional links, and shutting down
unreachable ports. The auto-negotiation mechanism and DLDP work together to ensure that
physical/logical unidirectional links can be detected and shut down, and to prevent failure of other
protocols such as STP. If both ends of a link are operating normally at the physical layer, DLDP detects
whether the link is correctly connected at the link layer and whether the two ends can exchange packets
properly. This is beyond the capability of the auto-negotiation mechanism at the physical layer.
How DLDP works
DLDP link states
A device is in one of these DLDP link states: Initial, Inactive, Active, Advertisement, Probe, Disable, and
DelayDown, as described in Table 10 .
Table 10 DLDP link states
State Indicates…
Initial DLDP is disabled.
Inactive DLDP is enabled, and the link is down.
Active DLDP is enabled and the link is up, or the neighbor entries have been cleared.
Advertisement
All neighbors are bi-directionally reachable or DLDP has been in active state for
more than five seconds. This is a relatively stable state where no unidirectional link
has been detected.
Probe
DLDP enters this state if it receives a packet from an unknown neighbor. In this
state, DLDP sends packets to check whether the link is unidirectional. As soon as
DLDP transits to this state, a probe timer starts and an echo timeout timer starts for
each neighbor to be probed.
Disable
A port enters this state when:
• A unidirectional link is detected.
• The contact with the neighbor in enhanced mode gets lost.
• In this state, the port does not receive or send packets other than DLDPDUs.
DelayDown
A port in the Active, Advertisement, or Probe DLDP link state transits to this state
rather than removes the corresponding neighbor entry and transits to the Inactive
state when it detects a port-down event. When a port transits to this state, the
DelayDown timer is triggered.
DLDP timers
Table 11 DLDP timers
DLDP timer Description
Active timer
Determines the interval for sending Advertisement packets with RSY tags, which
defaults to 1 second. By default, a device in the active DLDP link state sends one
Advertisement packet with RSY tags every second. The maximum number of
advertisement packets with RSY tags that can be sent successively is 5.
Advertisement timer
Determines the interval for sending common advertisement packets, which
defaults to 5 seconds.
Probe timer
Determines the interval for sending Probe packets, which defaults to 1 second. By
default, a device in the probe state sends one Probe packet every second. The
maximum number of Probe packets that can be sent successively is 10.