UDLD is a Layer 2 protocol enabled between adjacent switches. It uses MAC 01-00-0c-cc-cc-cc with Subnetwork Access Protocol (SNAP) High-Level Data Link Control (HDLC) protocol type 0x0111. UDLD packets contain information about sending the port’s device ID and port ID and the neighbor’s device ID and port ID. Neighbor devices with UDLD enabled send the same hello message. The link is bidirectional if devices on both sides receive each other’s UDLD packets. If the port does not see its own device and port ID in the incoming UDLD packets for a specific duration of time (timeout interval), the link is considered unidirectional and is disabled.
Using the default values for the advertisement timer (15 seconds) and hold timer (5 seconds), UDLD can take up to 20 seconds to respond to an error.


Normal-mode UDLD classifies a link as unidirectional if the received UDLD packets do not contain information that is correct for the neighbor device.

Aaggressive-mode UDLD puts ports into the errdisabled state if the relationship between two previously synchronized neighbors cannot be re-established.


Good to know:

⦁ By defaul UDLD is disabled on switch (can be enabled globally or per interface)
⦁ Both UDLD peers (switches) discover each other by exchanging special frames sent to well-known MAC address 01:00:0C:CC:CC:CC.
⦁ Using the default values for the advertisement timer (15 seconds) and hold timer (5 seconds), UDLD can take up to 20 seconds to respond to an error.
⦁ Aggrssive mode will change interface state to err-disabled after 8 reties

Configuration


To enble UDLD:

Switch(config)# interface f0/13
Switch(config-if)# udld port

To enable aggressive mode:

Switch(config)# interface f0/13
Switch(config-if)# udld port aggressive

To reset err-disabled interface:

Switch# udld reset

or

Switch(if-config)# shut
Switch(if-config)# no shut

Troubleshooting:

Switch(if-config)# show udld f0/1

Switch# debug udld events

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