Unidirectional Link Detection (UDLD)

“The purpose of the UDLD protocol is to detect the presence of anomalous conditions in the Layer 2 communication channel, while relying on the mechanisms defined by the IEEE in the 802.3 standard to properly handle conditions inherent to the physical layer.” –rfc5171

  • Compatible with existing HPE products:
    • Forward-then-verify: Packets are forwarded until the link is considered unidirectional
    • Verify-then-forward: Packets are not forwarded until the link has been determined to be bidirectional
  • Compatible with RFC5171-compliant devices:
    • Normal: Determines link unidirectionality but will not block the port
    • Aggressive: Once a port has been determined to be bidirectional and then becomes unidirectional, it will be blocked

NOTE The default UDLD mode is forward-then-verify.

Configuration Commands

Enable UDLD:

switch(config-if)# udld

Show commands to validate functionality:

show udld [interface IFACE]

Example Output

switch(config)# interface 1/1/1
switch(config-if)# udld
switch(config-if)# exit
show udld interface 1/1/1
Interface 1/1/1
 Config: enabled
 State: inactive
 Substate: uninitialized
 Link: unblock
 Version: aruba os
 Mode: forward then verify
 Interval: 7000 milliseconds
 Retries: 4
 Tx: 0 packets
 Rx: 0 packets, 0 discarded packets, 0 dropped packets
 Port transitions: 0

Expected Results

  1. Administrators can enable UDLD on an interface
  2. UDLD state should be “Unblocked | UDLD determined the link is bidirectional”

NOTE With SFP+ XCVERS, Aruba Switches automatically detect a broken bidirectional link, rendering the port into a down state.

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