When configuring DLDP, go to these sections
for information you are interested in:
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Overview
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DLDP Configuration
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DLDP Configuration Example
A special kind
of links, namely, unidirectional links, may occur in a network. When a
unidirectional link appears, the local device can receive packets from the peer
device through the link layer, but the peer device cannot receive packets from
the local device. Unidirectional link can cause problems such as loops in a Spanning
Tree Protocol (STP) enabled network.
Unidirectional links can be caused by
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Fiber cross-connection, as shown in Figure 1-1
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Fibers that are not connected or disconnected,
as shown in Figure 1-2, the hollow lines in which refer to fibers that are not connected
or disconnected.
Device link detection protocol (DLDP) can
detect the link status of an optical fiber cable or copper twisted pair (such
as super category 5 twisted pair). If DLDP finds a unidirectional link, it
disables the related port automatically or prompts you to disable it manually
according to the configurations, to avoid network problems.
Figure 1-1 Fiber
cross-connection
Figure 1-2 Fiber broken or not connected
DLDP provides the following features:
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As a link layer protocol, it works together with
the physical layer protocols to monitor the link status of a device.
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The auto-negotiation mechanism at the physical
layer detects physical signals and faults. DLDP identifies peer devices and
unidirectional links, and disables unreachable ports.
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Even if both ends of links can work normally at
the physical layer, DLDP can detect whether these links are connected correctly
and whether packets can be exchanged normally at both ends. However, the
auto-negotiation mechanism cannot implement this detection.
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In order for DLDP to detect fiber disconnection
in one direction, you need to configure the port to work in mandatory full duplex
mode at a mandatory rate.
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When the port determines the duplex mode and
speed through auto-negotiation, even if DLDP is enabled, it does not take
effect when the fiber in one direction is disconnected. In this case, the port
is considered down.
DLDP detects link status by exchanging the following
types of packets.
Table 1-1
DLDP packet types
|
DLDP packet type
|
Function
|
|
Advertisement
|
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.
|
|
RSY-Advertisement
packets (referred to as RSY packets hereafter)
|
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.
|
|
Flush-Advertisement packets (referred to
as flush packets hereafter)
|
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
|
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.
|
|
Echo
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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
|
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
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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).
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Recover Probe
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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.
|
|
Recover Echo
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Recover echo packets are response to
recover probe packets in the port auto-recovery mechanism. A link is
considered to restore to the bidirectional state if a port on one end sends a
recover probe packet, receives a recover echo packet, and the neighbor information
contained in the recover echo packet is consistent with that of the local
port.
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1)
If the DLDP-enabled link is up, DLDP sends DLDP
packets to the peer device, and analyzes/processes the DLDP packets received
from the peer device. DLDP packets sent in different DLDP states are of different
types.
Table 1-2
DLDP state and DLDP packet type
|
DLDP state
|
Type of the DLDP packets sent
|
|
Active
|
Advertisement packets, with the RSY flag
set or not set.
|
|
Advertisement
|
Advertisement packets
|
|
Probe
|
Probe packets
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2)
A DLDP packet received is processed as follows:
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In authentication mode, the DLDP packet is authenticated
and is then dropped if it fails the authentication.
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The packet is further processed, as described in
Table 1-3.
Table 1-3 The procedure to process a received
DLDP packet
|
Packet type
|
Processing procedure
|
|
Advertisement packet
|
Extracts neighbor information
|
If the corresponding neighbor entry does
not exist on the local device, DLDP creates the neighbor entry, triggers the
entry aging timer, and switches to the probe state.
|
|
If the corresponding neighbor entry
already exists on the local device, DLDP resets the aging timer of the entry.
|
|
Flush packet
|
Removes the neighbor entry from the local
device
|
|
Probe
packet
|
Sends echo
packets containing both neighbor and its own information to the peer
|
Creates
the neighbor entry if it does not exist on the local device.
|
|
Resets the
aging timer of the entry if the neighbor entry already exists on the local
device.
|
|
Echo
packet
|
Checks to
see if the local device is in the probe state
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No
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Drops the
echo packet
|
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Yes
|
Checks to
see if the neighbor information contained in the packet is the same as that
on the local device
|
No
|
Drops the
echo packet
|
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Yes
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Sets the flag
bit of the neighbor to bidirectional link
|
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If all neighbors are in the bidirectional
link state, DLDP switches from the probe state to the advertisement state,
and sets the echo waiting timer to 0.
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3)
If no echo packet is received from the neighbor,
DLDP performs the following processing:
Table 1-4
Processing procedure when no echo packet is
received from the neighbor
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No echo packet received from the neighbor
|
Processing procedure
|
|
In normal mode, no echo packet is
received when the echo waiting timer expires.
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DLDP switches to the disable state,
outputs log and tracking information, and sends flush packets. Depending on
the user-defined DLDP down mode, DLDP disables the local port automatically
or prompts you to disable the port manually. DLDP sends RSY messages and removes
the corresponding neighbor entries.
|
|
In enhanced mode, no echo packet is
received when the enhanced timer expires
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A link can be in one of these DLDP states:
initial, inactive, active, advertisement, probe, disable, and delaydown.
Table 1-5 DLDP status
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Status
|
Description
|
|
Initial
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Initial status before DLDP is 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 directions, or DLDP remains in active state for more than five seconds
and enters this status. It is a stable state where no unidirectional link is
found
|
|
Probe
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DHCP sends packets to check whether the
link is a unidirectional. 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. In this case, DLDP sends
and receives only recover probe packets and recover echo packets.
|
|
DelayDown
|
When a device in the active,
advertisement, or probe DLDP state receives a port down message, it does not
removes the corresponding neighbor immediately, neither does it changes to
the inactive state. Instead, it changes to the delaydown state first.
When a device changes to the delaydown
state, the related DLDP neighbor information remains, and the DelayDown timer
is triggered. After the DelayDown timer expires, the DLDP neighbor information
is removed.
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Table 1-6 DLDP timers
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Timer
|
Description
|
|
Advertisement sending timer
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Interval between sending advertisement
packets, which can be configured on a command line interface.
By default, the timer length is 5
seconds.
|
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Probe sending timer
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The interval is 0.5 seconds. In the probe
state, DLDP sends two probe packets in a second.
|
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Echo waiting timer
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It is enabled when DLDP enters the probe
state. The echo waiting timer length is 10 seconds.
If no echo packet is received from the
neighbor when the Echo waiting timer expires, the state of the local end is
set to unidirectional link (one-way audio) and the state machine turns into
the disable state. DLDP outputs log and tracking information, sends
flush packets. Depending on the user-defined DLDP down mode, DLDP disables
the local port automatically or prompts you to disable the port manually. At
the same time, DLDP deletes the neighbor entry.
|
|
Entry aging timer
|
When a new neighbor joins, a neighbor
entry is created and the corresponding entry aging timer is enabled
When an advertisement packet is received
from a neighbor, the neighbor entry is updated and the corresponding entry
aging timer is updated
In the normal mode, if no packet is
received from the neighbor when the entry aging timer expires, DLDP sends an
advertisement packet with an RSY tag, and deletes the neighbor entry.
In the enhanced mode, if no packet is
received from the neighbor when the entry aging timer expires, DLDP enables
the enhanced timer
The entry aging timer length is three
times the advertisement timer length.
|
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Enhanced timer
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In the enhanced mode, if no packet is
received from the neighbor when the entry aging timer expires, DLDP enables
the enhanced timer for the neighbor. The enhanced timer length is 10 seconds
The enhanced timer then sends one probe
packet every second and eight packets successively to the neighbor.
If no echo packet is received from the
neighbor when the enhanced timer expires, the state of the local end is set
to unidirectional communication state and the state machine turns into the disable
state. DLDP outputs log and tracking information and sends flush packets.
Depending on the user-defined DLDP down mode, DLDP disables the local port automatically
or prompts you to disable the port manually. Meanwhile, DLDP deletes the
neighbor entry.
|
|
DelayDown timer
|
When a device in the active,
advertisement, or probe DLDP state receives a port down message, it does not
removes the corresponding neighbor immediately, neither does it changes to
the inactive state. Instead, it changes to the delaydown state first.
When a device changes to the delaydown
state, the related DLDP neighbor information remains, and the DelayDown timer
is triggered. The DelayDown timer is configurable and ranges from 1 to 5
seconds.
A device in the delaydown state only
responds to port up messages.
A device in the delaydown state resumes
its original DLDP state if it receives a port up message before the delaydown
timer expires. Otherwise, it removes the DLDP neighbor information and changes
to the inactive state.
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DLDP can operate in two modes: normal and
enhanced.
Table 1-7 DLDP operating mode and
neighbor entry aging
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DLDP operating mode
|
DLDP detects whether neighbors exist
or not when neighbor tables are aging
|
The entry aging timer is enabled or
not during neighbor entry aging
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The enhanced timer is enabled or not
when the entry aging timer expires
|
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Normal mode
|
No
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Yes (The neighbor entry ages out after
the entry aging timer expires)
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No
|
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Enhanced mode
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Yes
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Yes (The enhanced timer is enabled after
the entry aging timer expires)
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Yes (When the enhanced timer expires, the
state of the local end is set to unidirectional link, and the neighbor entry
is aged out.)
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1.2.5 DLDP Neighbor State
A DLDP neighbor can be in one of these two
states: two way and unknown. You can check the state of a DLDP neighbor by
using the display dldp command.
Table 1-8
Description on the two DLDP neighbor states
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DLDP neighbor state
|
Description
|
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two way
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The link to the neighbor operates
properly.
|
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unknown
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The device is detecting the neighbor and
the neighbor state is unknown.
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If the shutdown mode of a port is set to
auto shutdown, the port is set to the DLDP down state when DLDP detects the
link connecting to the port is a unidirectional link. A port in DLDP down state
does not forward service packets or receive/send protocol packets except
DLDPDUs.
A port in the DLDP down state recovers when
the corresponding link recovers. A port in the DLDP down state sends recover
probe packets periodically. On receiving a correct recover echo packet (which
means that the unidirectional link is restored to a bidirectional link), it is
brought up by DLDP. The detailed process is as follows.
1)
A port in the DLDP down state sends a recover
probe packet every 2 seconds. Recover probe packets carry only the local port
information.
2)
Upon receiving a recover probe packet, the peer
end responds with a recover echo packet.
3)
Upon receiving a recover echo packet, the local
end checks to see if the neighbor information carried in the recover echo
packet is consistent with that of the local port. If yes, the link between the
local port and the neighbor is considered to be recovered to bidirectional, the
port changes from the disable state to the active state, and neighboring
relationship is reestablished between the local port and the neighbor.
Only ports in the
DLDP down state can send and process recover probe packets and recover echo
packets. The auto-recovery mechanism does apply to ports that are shut down
manually.
Follow these steps
to perform basic DLDP configuration:
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To do …
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Use the command …
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Remarks
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|
Enter system view
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system-view
|
—
|
|
Enable DLDP
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Enable
DLDP globally
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dldp enable
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Required.
By default,
DLDP is disabled.
|
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Enable DLDP on a port
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Enter Ethernet port view
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interface interface-type
interface-number
|
|
Enable DLDP
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dldp enable
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Set the authentication mode and password
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dldp authentication-mode { none | simple simple-password | md5 md5-password
}
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Optional.
By default, the authentication mode is none.
|
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Set the interval of sending DLDP packets
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dldp interval timer-value
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Optional.
By default, the interval is 5 seconds.
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Set the delaydown timer
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dldp delaydown-timer delaydown-time
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Optional
By default, the delaydown timer expires
after 1 second it is triggered.
|
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Set the DLDP handling mode when an
unidirectional link is detected
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dldp unidirectional-shutdown { auto | manual }
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Optional.
By default, the handling mode is auto.
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Set the DLDP operating mode
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dldp work-mode { enhance | normal }
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Optional.
By default, DLDP works in normal mode.
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Note the following when performing basic
DLDP configuration.
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DLDP works only when the link is up.
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To ensure unidirectional links can be detected,
make sure DLDP is enabled on both sides; and the interval for sending
advertisement packets, authentication mode, and password are the same on both
sides.
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The interval for sending advertisement packets ranges
from 1 to 100 seconds and defaults to 5 seconds. You can adjust this setting as
needed to enable DLDP to respond in time to link failures. If the interval is
too long, STP loops may occur before unidirectional links are terminated; if
the interval is too short, network traffic may increase in vain and available
bandwidth decreases. Normally, the interval is shorter than one-third of the
STP convergence time, which is generally 30 seconds.
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DLDP does not process any LACP event, and treats
each link in the aggregation group as independent.
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When connecting two DLDP-enabled devices, make
sure the software running on them is of the same version. Otherwise, DLDP may
operate improperly.
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When you use the dldp enable/dldp disable command
in system view to enable/disable DLDP on all optical ports of the switch, the
configuration takes effect on the existing optical ports, instead of those
added subsequently.
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Make sure the authentication mode and password
configured on both sides are the same for DLDP to operate properly.
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When DLDP works in enhanced mode, the system can
identify two types of unidirectional links: one is caused by fiber
cross-connection and the other is caused by one fiber being not connected or
being disconnected.
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When DLDP works in normal mode, the system can
identify unidirectional links caused by fiber cross-connection.
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When the device is busy with services and the
CPU utilization is high, DLDP may issue mistaken reports. You are recommended
to configure the operating mode of DLDP as manual after unidirectional links
are detected, so as to reduce the influence of mistaken reports.
You can reset the DLDP state for the ports
shut down by DLDP due to unidirectional links to enable DLDP detection again.
This function is
only applicable to ports that are in DLDP down state.
Follow
these steps to reset DLDP state:
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To do …
|
Use the command …
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Remarks
|
|
Reset DLDP state for all the ports shut
down by DLDP
|
system-view
|
Select either of the two.
|
|
dldp reset
|
|
Reset the DLDP state for a port shut down
by DLDP
|
interface interface-type interface-number
|
|
dldp reset
|
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To do …
|
Use the command …
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Remarks
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Display the DLDP configuration of a unit
or a port
|
display dldp { unit-id | interface-type interface-number }
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Available in any view.
|
I. Network requirements
As shown in Figure 1-3,
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Switch A and Switch B are connected through two
pairs of fibers. Both of them support DLDP. All the ports involved operate in
mandatory full duplex mode, with their rates all being 1,000 Mbps.
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Suppose the fibers between Switch A and Switch B
are cross-connected. DLDP disconnects the unidirectional links after detecting them.
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After the fibers are connected correctly, the
ports shut down by DLDP are restored.
II. Network diagram
Figure 1-3 Network
diagram for DLDP configuration
III. Configuration procedure
1)
Configure Switch A
# Configure the ports to work in mandatory
full duplex mode at a rate of 1000 Mbps.
<SwitchA> system-view
[SwitchA] interface gigabitethernet 1/0/50
[SwitchA-GigabitEthernet1/0/50]
duplex full
[SwitchA-GigabitEthernet1/0/50] speed
1000
[SwitchA-GigabitEthernet1/0/50] quit
[SwitchA] interface gigabitethernet 1/0/51
[SwitchA-GigabitEthernet1/0/51]
duplex full
[SwitchA-GigabitEthernet1/0/51] speed
1000
[SwitchA-GigabitEthernet1/0/51] quit
# Enable DLDP globally
[SwitchA] dldp enable
# Set the interval between sending DLDP
packets to 15 seconds.
[SwitchA] dldp interval 15
# Configure DLDP to work in enhanced mode
[SwitchA] dldp work-mode enhance
# Set the DLDP handling mode for
unidirectional links to auto.
[SwitchA] dldp unidirectional-shutdown
auto
# Display the DLDP state
[SwitchA] display dldp 1
When two switches are connected through
fibers in a crossed way, two or three ports may be in the disable state, and
the rest in the inactive state.
When a fiber is connected to a device
correctly on one end with the other end connected to no device:
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If the device operates in the normal DLDP mode,
the end that receives optical signals is in the advertisement state; the other
end is in the inactive state.
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If the device operates in the enhance DLDP mode,
the end that receives optical signals is in the disable state; the other end is
in the inactive state.
# Restore the ports shut down by DLDP
[SwitchA] dldp reset
2)
Configure Switch B
The configuration of Switch B is the same
to that of Switch A and is thus omitted.
