1. Technology Brief: D-Link Unidirectional Link Detection (DULD)
Ethernet Switch Product Department
Sept. 2011 HQ

2. Concept of D-Link Unidirectional Link Detection
Unidirectional link detection provides discovery mechanism based on IEEE 802.3ah to discovery its neighbor.
If the discovery can complete in configured discovery time, it concludes the link is bidirectional. Otherwise, it starts detecting task to detect the unidirectional link status.
Once the local port detected the link is unidirectional, it sends link fault message to remote port and sets the link to fault status. Remote port that received link fault message sets its link status to fault too.
Unidirectional OAM Operation
Link Fault Message TX
Local Port
Remote
Port RX
Fault Status
Fault Status
Unidirectional Link
NOTE: Only when both local and remote ports have the unidirectional link detection capability, the
detection function is enabled.

3. Problem:
STP relies in its operation on reception and transmission of the Bridge Protocol Data Units (BPDUs). If the STP process that runs on the switch with a blocking port stops receiving BPDUs from its upstream (designated) switch on the port, STP eventually ages out the STP information for the port and moves it to the forwarding state.
This creates a forwarding loop or STP loop. Packets start to cycle indefinitely along the looped path, and consumes more and more bndwidth.
Root Bridge
Designated Port
Loop
BPDU
Forwarding State
Blocking Port

4. When unidirectional link happened, Shutdown the port
What Is Solved?
How is it possible for the switch to stop receiving BPDUs while the port is up? The reason is unidirectional link. A link is considered unidirectional when this occurs:
The link is up on both sides of the connection.
The local side is not receiving the packets sent by the remote side while remote side receives packets sent by local side
In order to detect the unidirectional links before the forwarding loop is created, D-Link Unidirectional Link Detection (DULD) is necessary.
Root Bridge
Designated Port
When unidirectional link happened, Shutdown the port

5. Local Detection TLV: Capability Advertisement
Understanding DULD
DULD (D-Link Unidirectional Link Detection) utilize OAM 802.3ah.
Unidirectional Link Detection needs both local and remote support it.
The advertisement is based on the extended OAM discovery. OAM discovery is initiated by active port. Once OAM is enabled on an active port, it sends information OAMPDU to remote peer.
The unidirectional link detection capability is contained in the local information TLV of the information OAMPDU.
The active port means IEEE 802.3ah OAM port mode is set to active mode: config ethernet_oam ports [<portlist> | all] mode [active | passive] | state [enable | disable] |
OAM
Discovery
Info OAMPDU TX RX
Active Port
Local Detection TLV: Capability Advertisement
Passive
Port

6. How Unidirectional Link Detection Works?
DULD makes use of Information OAMPDU as the detection message.
One DULD detection message will be sent per second, totally 5 times. The other party must echo back when it receives a DULD detection message.
If the local port receives any detection message which includes itself information, it concludes the link is bidirectional. Otherwise, if there is no echoed detection message after sending the detection messages for 5 times, it concludes the link is unidirectional.
Info OAMPDU: 5 times TX RX
No Echo
Local Port
Detection Message
Remote Port
Result: Unidirectional Link

7. How OAM Discovery Works?
OAM discovery is the first phase process of Ethernet OAM. It is to advertise OAM capability of both parties.
OAM discovery makes use of the Information OAMPDU.
The DULD capability is contained in the Local detection TLV of the information OAMPDU.
If OAM discovery cannot complete within configured discovery time, and the port received Information OAMPDU indicating its peer supports DULD, DULD detection will start immediately.
Once OAM discovery completes, OAM association of both ends is formed.
OAM
Discovery
Info OAMPDU TX RX
Active Port
Local Detection TLV: Capability Advertisement
Passive
Port
Result: Bidirectional Link

8. How Link Fault Message Works?
Once a link is concluded as unidirectional, the port sends a link fault message to the other party, and sets the link to fault status.
The other party, while receiving link fault message, set the link to fault status too.
During discovery, If the TX is break, both active and passive cannot receive the information OAMPDU, so that the unidirectional link detection cannot be done.
Suggest to configure the port to active mode at both local and remote, thus the link fault message from the passive port can be received by active port.
Info OAMPDU TX RX
Fault status
Active Port
Link Fault Message
Active/Passive Port
Result: Unidirectional link

9. What is the result of DULD?
Once unidirectional link is detected, the port will enter link fault status.
If the port’s unidirectional OAM operation mode is configured to shutdown, the port will be disabled automatically.
Otherwise, if a port’s unidirectional OAM operation mode is normal, only log the unidirectional link event.
If the unidirectional OAM operation mode of the port is normal, this port can recover automatically when it re-linkup.
If the mode is shutdown, this port will be disabled, and user must enable this port manually to recover this port link status.
Info OAMPDU: TX RX
Shutdown port or Log
Local Port
Detection Message
Remote Port

10. Detection Message Format
Detection Message is contained in information OAMPDU, the format as follows:
Info OAMPDU:
Detect TLV Field:
Destination Address
Source Address
Length/Type
Subtype
Flags
Code = 0x00
Data/Pad
FCS
Type = 0xFE
Length
OUI = 0x00055D
Detection version = 0x01
Operation code
Link Fault
Request sequence number
Local Port ID
Remote Port ID
Info TLV:
Local Info TLV
Remote Info TLV
Detection TLV
Other Info TLV
MAC Address
Port Number

11. Local Information TLV Format
Info type=0x01
Info Length
OAM version
Revision
State
OAM Configuration
OAMPDU Configuration
OUI = 0x00055D
Vendor Specific Info
The unidirectional link detection capability is contained in the local information TLV of the information OAMPDU
The D-Link OUI is D.
The 32 bits Vendor Specific Info is define as following:
Bit 0: Unidirectional link detection capability
Bit 1-31: Reserved
When a port received the information OAMPDU, it checks the unidirectional link detection capability bit. If this bit is set and supports unidirectional link detection too, the unidirectional link detection operation status is enabled, otherwise it is disabled.
Only if the operation status is enabled, detection can be started

12. DULD Configuration Example
Test Setup:
- DUT: DES Firmware version:
Configuration:
For DES-3528A and DES-3528B
config ethernet_oam ports state enable
config duld ports mode shutdown state enable

13. DULD Configuration Verification
UDLD Status
Port Status
DES-3528_A:admin#show duld ports 25-26
Command: show duld ports 25-26
Port Admin State Oper Status Mode Link Status Discovery Time(Sec)
Enabled Enabled Shutdown Bidirectional 5
Enabled Enabled Shutdown Bidirectional 5
DES-3528_B:admin#show duld ports 25-26
DES-3528_A:admin#show ports 25-26
Command: show ports 25-26
Port State/ Settings Connection Address
MDIX Speed/Duplex/FlowCtrl Speed/Duplex/FlowCtrl Learning
25 (C) Enabled Auto/Disabled Link Down Enabled
Auto
25 (F) Enabled Auto/Disabled M/Full/None Enabled
26 (C) Enabled Auto/Disabled Link Down Enabled
26 (F) Enabled Auto/Disabled M/Full/None Enabled
DES-3528_B:admin#show ports 25-26
25 (C) Enabled Auto/Disabled Link Down Enabled
25 (F) Enabled Auto/Disabled M/Full/None Enabled
26 (C) Enabled Auto/Disabled Link Down Enabled
26 (F) Enabled Auto/Disabled M/Full/None Enabled

14. DULD Fault Scenario The unidirectional issue is happening.
DES-3528_A:admin#show log severity 6
Command: show log severity 6
Index Date Time Level Log Text
:47:56 INFO(6) Port 26 link down
:47:56 INFO(6) Port 25 link down
:47:56 INFO(6) Port: 25 is unidirectional
:47:56 INFO(6) Port: 26 is unidirectional
:47:43 INFO(6) MSTP(4):Spanning Tree port role change (Instance:0, Port:26) DisabledPort->DesignatedPort
:47:43 INFO(6) MSTP(4):Spanning Tree port role change (Instance:0, Port:25) DisabledPort->DesignatedPort
:47:43 INFO(6) Port 26 link up, 1000Mbps FULL duplex
:47:43 INFO(6) Port 25 link up, 1000Mbps FULL duplex

15. DULD Fault Status Verification
UDLD Status
Port Status
DES-3528_A:admin#show duld ports 25-26
Command: show duld ports 25-26
Port Admin State Oper Status Mode Link Status Discovery
Time(Sec)
Enabled Disabled Shutdown TX Fault
Enabled Disabled Shutdown TX Fault
DES-3528_B:admin#show duld ports 25-26
Port Admin State Oper Status Mode Link Status Discovery Time(Sec)
DES-3528_A:admin#show ports 25-26
Command: show ports 25-26
Port State/ Settings Connection Address
MDIX Speed/Duplex/FlowCtrl Speed/Duplex/FlowCtrl Learning
25 (C) Enabled Auto/Disabled Err-Disabled Enabled
Auto
25 (F) Enabled Auto/Disabled Err-Disabled Enabled
26 (C) Enabled Auto/Disabled Err-Disabled Enabled
26 (F) Enabled Auto/Disabled Err-Disabled Enabled
DES-3528_B:admin#show ports 25-26
Port State/ Settings Connection Address