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The "%UDLD-3-AGGRDISABLE" error message is displayed in the logs of a Catalyst 6500 series switch
This message indicates that a fault has been detected in an Ethernet port connection, and that the port has been disabled in order to prevent other protocols from malfunctioning:
%UDLD-3-AGGRDISABLE: Neighbor(s) of port [dec]/[dec] disappeared on bidirectional link. Port disabled
The UniDirectional Link Detection (UDLD) messages (packets) on a bidirectional link have stopped and, as a precaution, the port is disabled in order to prevent a spanning tree loop. The [dec]/[dec] portion of this error message is the module number/port number.
The switch periodically transmits UDLD messages (packets) to neighbor devices on ports that have UDLD enabled. If the messages are echoed back to the sender within a specific time frame and they lack a specific acknowledgment (echo), the link is flagged as unidirectional and the port is shut down. Devices on both ends of the link must support UDLD in order for the protocol in order to successfully identify and disable unidirectional links.
With UDLD aggressive mode enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops to receive UDLD packets, UDLD tries to re-establish the connection with the neighbor. After eight failed re-tries, the port is put into errdisable state.
Complete these steps in order to resolve this issue:
Verify that all of the fiber strand pairs or copper wire pairs are connected correctly and that the transmit (Tx) and receive (Rx) fiber connectors are bundled together.
Check that the port functions normally and that the hardware counters operate properly.
If no apparent misconnection or malfunction is found, try to re-enable the port and disable the aggressive mode.
Make sure that the port is not connected to a hub, media converter, or similar device.
The ports and links on both the switches can be troubleshooted if you issue the show counters interface command on the ports and examine which error counters have high counts. This points out what caused the traffic to stop to flow in one direction, which results in the link going down. Also check the Gigabit Interface Converters (GBICs) and physical links.
Note: When a switch detects the uni-directional link through its UDLD process, only the malfunctioning ports are turned into the err-disabled.