Sometimes, unidirectional links may appear in networks. On a unidirectional link, one end can receive packets from the other end but the other end cannot. Unidirectional links result in problems such as loops in an STP-enabled network.
The Device Link Detection Protocol (DLDP) can detect the link status of a fiber cable or twisted pair. On detecting a unidirectional link, DLDP can shut down the related port automatically or prompt users to take measures as configured to avoid network problems.As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor the link status of a switch. The auto-negotiation mechanism provided by physical layer protocols detects physical signals and faults. DLDP, however, performs operations such as identifying peer switches, detecting unidirectional links, and shutting down unreachable ports. The cooperation of physical layer protocols and DLDP ensures that physical/logical unidirectional links be detected and shut down. For a link with the switches on the both sides of it operating properly, DLDP checks whether the cable is connected correctly and whether packets can be exchanged between the two switches. This is beyond the capability of the auto-negotiation mechanism at the physical layer.
DLDP can operate in two modes: normal mode and enhanced mode, as described below.
The Device Link Detection Protocol (DLDP) can detect the link status of a fiber cable or twisted pair. On detecting a unidirectional link, DLDP can shut down the related port automatically or prompt users to take measures as configured to avoid network problems.As a data link layer protocol, DLDP cooperates with physical layer protocols to monitor the link status of a switch. The auto-negotiation mechanism provided by physical layer protocols detects physical signals and faults. DLDP, however, performs operations such as identifying peer switches, detecting unidirectional links, and shutting down unreachable ports. The cooperation of physical layer protocols and DLDP ensures that physical/logical unidirectional links be detected and shut down. For a link with the switches on the both sides of it operating properly, DLDP checks whether the cable is connected correctly and whether packets can be exchanged between the two switches. This is beyond the capability of the auto-negotiation mechanism at the physical layer.
DLDP can operate in two modes: normal mode and enhanced mode, as described below.
l In normal DLDP mode, when an entry timer expires, the switch removes the corresponding neighbor entry and sends an Advertisement packet with RSY tag.
l In enhanced DLDP mode, when an entry timer expires, the Enhanced timer is triggered and the switch sends up to eight Probe packets at a frequency of one packet per second to test the neighbor. If no Echo packet is received from the neighbor when the Echo timer expires, the switch transits to the Disable state.
Configuration procedure
1) Configuration on Device A
# Enable DLDP on GigabitEthernet2/0/1 and GigabitEthernet 2/0/2 separately.
<DeviceA> system-view
[DeviceA] interface gigabitethernet 2/0/1
[DeviceA-GigabitEthernet2/0/1] dldp enable
[DeviceA-GigabitEthernet2/0/1] quit
[DeviceA] interface gigabitethernet 2/0/2
[DeviceA-GigabitEthernet2/0/2] dldp enable
[DeviceA-GigabitEthernet2/0/2] quit
# Set the interval for sending Advertisement packets to 6 seconds.
[DeviceA] dldp interval 6
# Set the DelayDown timer to 2 seconds.
[DeviceA] dldp delaydown-timer 2
# Set the DLDP mode as enhanced mode.
[DeviceA] dldp work-mode enhance
# Set the port shutdown mode as auto mode.
[DeviceA] dldp unidirectional-shutdown auto
# Enable DLDP globally.
[DeviceA] dldp enable
2) Configuration on Device B
Configure Device B as you configure Device A.
3) Verifying the configurations
You can use the display dldp command to display the DLDP configuration information on ports.
# Display the DLDP configuration information on all the DLDP-enabled ports of Device A.
[DeviceA] display dldp
DLDP global status : enable
DLDP interval : 6s
DLDP work-mode : enhance
DLDP authentication-mode : none
DLDP unidirectional-shutdown : auto
DLDP delaydown-timer : 2s
The number of enabled ports is 2.
Interface GigabitEthernet2/0/1
DLDP port state : disable
DLDP link state : down
The neighbor number of the port is 0.
Interface GigabitEthernet2/0/2
DLDP port state : disable
DLDP link state : down
The neighbor number of the port is 0.
The output information indicates that both GigabitEthernet2/0/1 and GigabitEthernet2/0/2 are in Disable state and the links are down, which means unidirectional links are detected and the two ports are thus shut down.
Correct the fiber connections after detecting the problem, and perform the following operations:
# Reset DLDP state for the ports shut down by DLDP.
[DeviceA] dldp reset
# Display the DLDP configuration information on all the DLDP-enabled ports of Device A.
[DeviceA] display dldp
DLDP global status : enable
DLDP interval : 6s
DLDP work-mode : enhance
DLDP authentication-mode : none
DLDP unidirectional-shutdown : auto
DLDP delaydown-timer : 2s
The number of enabled ports is 2.
Interface GigabitEthernet2/0/1
DLDP port state : advertisement
DLDP link state : up
The neighbor number of the port is 1.
Neighbor mac address : 0000-0000-0101
Neighbor port index : 59
Neighbor state : two way
Neighbor aged time : 11
Interface GigabitEthernet2/0/2
DLDP port state : advertisement
DLDP link state : up
The neighbor number of the port is 1.
Neighbor mac address : 0000-0000-0102
Neighbor port index : 59
Neighbor state : two way
Neighbor aged time : 11
The output information indicates that both GigabitEthernet2/0/1 and GigabitEthernet2/0/2 are in the Advertisement state and the links are up, which means unidirectional links are not detected and the two ports are restored.
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