Hi Andrew,
1- Yes UDLD is by default enabled on RSTP protocol.
2- when it is failing very often kindly check the cable connecting between the ends once.
3- Please find below way the UDLD works:-
In order to detect the unidirectional links before the forwarding loop is created, Cisco designed and implemented the UDLD protocol.
UDLD is a Layer 2 (L2) protocol that works with the Layer 1 (L1) mechanisms to determine the physical status of a link. At Layer 1, auto-negotiation takes care of physical signaling and fault detection. UDLD performs tasks that auto-negotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected ports. When you enable both auto-negotiation and UDLD, Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.
UDLD works by exchanging protocol packets between the neighboring devices. In order for UDLD to work, both devices on the link must support UDLD and have it enabled on respective ports.
Each switch port configured for UDLD sends UDLD protocol packets that contain the port's own device/port ID, and the neighbor's device/port IDs seen by UDLD on that port. Neighboring ports should see their own device/port ID (echo) in the packets received from the other side.
If the port does not see its own device/port ID in the incoming UDLD packets for a specific duration of time, the link is considered unidirectional.
This echo-algorithm allows detection of these issues:
- Link is up on both sides, however, packets are only received by one side.
- Wiring mistakes when receive and transmit fibers are not connected to the same port on the remote side.
Once the unidirectional link is detected by UDLD, the respective port is disabled and this message is printed on the console:UDLD-3-DISABLE: Unidirectional link detected on port 1/2. Port disabledPort shutdown by UDLD remains disabled until it is manually reenabled, or until errdisable timeout expires (if configured).
UDLD can operate in two modes: normal and aggressive.In normal mode, if the link state of the port was determined to be bi-directional and the UDLD information times out, no action is taken by UDLD. The port state for UDLD is marked as undetermined. The port behaves according to its STP state.In aggressive mode, if the link state of the port is determined to be bi-directional and the UDLD information times out while the link on the port is still up, UDLD tries to re-establish the state of the port. If not successful, the port is put into the errdisablestate.Aging of UDLD information happens when the port that runs UDLD does not receive UDLD packets from the neighbor port for duration of hold time. The hold time for the port is dictated by the remote port and depends on the message interval at the remote side. The shorter the message interval, the shorter the hold time and the faster the detection. Recent implementations of UDLD allow configuration of message interval.UDLD information can age out due to the high error rate on the port caused by some physical issue or duplex mismatch. Such packet drop does not mean that the link is unidirectional and UDLD in normal mode will not disable such link.It is important to be able to choose the right message interval in order to ensure proper detection time. The message interval should be fast enough to detect the unidirectional link before the forwarding loop is created, however, it should not overload the switch CPU. The default message interval is 15 seconds, and is fast enough to detect the unidirectional link before the forwarding loop is created with default STP timers. The detection time is approximately equal to three times the message interval.For example: Tdetection ~ message_interval x3This is 45 seconds for the default message interval of 15 seconds.It takes Treconvergence=max_age + 2x forward_delay for the STP to reconverge in case of unidirectional link failure. With the default timers, it takes 20+2x15=50 seconds.It is recommended to keep Tdetection < Treconvergence by choosing an appropriate message interval.In aggressive mode, once the information is aged, UDLD will make an attempt to re-establish the link state by sending packets every second for eight seconds. If the link state is still not determined, the link is disabled.Aggressive mode adds additional detection of these situations:
- The port is stuck (on one side the port neither transmits nor receives, however, the link is up on both sides).
- The link is up on one side and down on the other side. This is issue might be seen on fiber ports. When transmit fiber is unplugged on the local port, the link remains up on the local side. However, it is down on the remote side.
Most recently, fiber FastEthernet hardware implementations have Far End Fault Indication (FEFI) functions in order to bring the linkdown on both sides in these situations. On Gigabit Ethernet, a similar function is provided by link negotiation. Copper ports are normally not susceptible to this type of issue, as they use Ethernet link pulses to monitor the link. It is important to mention that in both cases, no forwarding loop occurs because there is no connectivity between the ports. If the link is up on one side and down on the other, however, blackholing of traffic might occur. Aggressive UDLD is designed to prevent this.
HTH
Regards
Inayath
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