3Com 5500 SI - Switch - Stackable, Configuration Manual

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90 C
HAPTER
5: DLDP C
ONFIGURATION
DLDP provides the following features:
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As a link layer protocol, it works together with the physical layer protocol to
monitor the link status of a device.
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While the auto-negotiation mechanism on the physical layer detects physical
signals and faults; DLDP identifies peer devices and unidirectional links, and
disables unreachable ports.
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The auto-negotiation mechanism and DLDP, when enabled, work together to
detect and disable physical and logical unidirectional links, and to prevent the
failure of other protocols, such as STP (Spanning Tree Protocol).
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Even if the links of both ends can normally operate individually on the physical
layer, DLDP can detect (at the link layer) if these links are set up correctly and
packets can be exchanged normally between the two ends. This cannot be
implemented by the auto-negotiation mechanism.
DLDP Fundamentals DLDP status
DLDP may be in one of the six states: initial, inactive, active, advertisement, probe and
disable.
DLDP timers
DLDP works with the following timers:
Table 71
DLDP status
Status Description
Initial DLDP is not enabled.
Inactive DLDP is enabled but the corresponding link is down
Active DLDP is enabled and the link is up, or an neighbor entry is cleared
Advertisement All neighbors communicate normally in both direction, or DLDP remains
in active status for more than five seconds and enters this status. It is a
stable status when no unidirectional link is found
Probe DHCP sends packets to check if it is a unidirectional link. It enables the
probe sending timer and an echo waiting timer for each target neighbor.
Disable DLDP detects a unidirectional link, or finds (in enhanced mode) that a
neighbor disappears. At this time, DLDP does not receive or send DLDP
packets.
Table 72
DLDP timers
Timer Description
Advertisement sending
timer Time interval for sending advertisement packets, which can be
configured with a particular command.By default, the time interval is 10
seconds.
Probe sending timer The time interval is 1 second. In probe status, DLDP sends two probe
packets every second.
Echo waiting timer It is enabled when DLDP enters probe status. The timeout time is 10
seconds.If no echo packet is received from the neighbor when the Echo
waiting timer expires, the local end is set to unidirectional
communication status and the state machine turns into disable status.
DLDP outputs log and tracking information, sends flush packets.
Depending on the user-defined DLDP down mode, DLDP disables the
local port automatically or prompt the user to disable the port manually.
At the same time, DLDP deletes the neighbor entry.