H3C Technologies H3C S12500 Series Switches - Manual (Page 47)

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As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor link status. While
the auto-negotiation mechanism provided by the physical layer detects physical signals and faults, DLDP
performs operations such as identifying peer switches, detecting unidirectional links, and shutting down
unreachable ports. The auto-negotiation mechanism and DLDP work together to make sure 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 correctly at the physical layer, DLDP detects
whether the link is correctly connected at the link layer and whether the two ends can exchange packets
correctly. This is beyond the capability of auto-negotiation mechanisms of the physical layer.
How DLDP works
DLDP link states
A switch is in one of these DLDP link states: Initial, Inactive, Active, Advertisement, Probe, Disable, and
DelayDown.
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 5 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 transitions 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 transitions
to this state rather than removes the corresponding neighbor entry and
transitions to the Inactive state when it detects a port-down event. When
a port transitions to this state, the DelayDown timer is triggered.
DLDP timers
Table 11 DLDP timers
DLDP timer Description
Active timer
Determines the interval to send Advertisement packets with RSY tags,
which defaults to 1 second. By default, a switch 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.