The Cisco-proprietary UDLD protocol allows devices connected through fiber-optic or copper (for example, Category 5 cabling) Ethernet cables connected to LAN ports to monitor the physical configuration of the cables and detect when a unidirectional link exists. When a unidirectional link is detected, UDLD shuts down the affected LAN port and alerts the user. Unidirectional links can cause a variety of problems, including spanning tree topology loops.
UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected LAN ports. When you enable both autonegotiation and UDLD, Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.
A unidirectional link occurs whenever traffic transmitted by the local device over a link is received by the neighbor but traffic transmitted from the neighbor is not received by the local device. If one of the fiber strands in a pair is disconnected, as long as autonegotiation is active, the link does not stay up. In this case, the logical link is undetermined, and UDLD does not take any action. If both fibers are working normally at Layer 1, then UDLD at Layer 2 determines whether those fibers are connected correctly and whether traffic is flowing bidirectionally between the correct neighbors. This check cannot be performed by autonegotiation, because autonegotiation operates at Layer 1.
LAN ports with UDLD enabled periodically transmit UDLD packets to neighbor devices. If the packets are echoed back within a specific time frame and they are lacking a specific acknowledgment (echo), the link is flagged as unidirectional and the LAN port is shut down. Devices on both ends of the link must support UDLD in order for the protocol to successfully identify and disable unidirectional links.
Note By default, UDLD is locally disabled on copper LAN ports to avoid sending unnecessary control traffic on this type of media since it is often used for access ports.
Figure 12-1 shows an example of a unidirectional link condition. Switch B successfully receives traffic from Switch A on the port. However, Switch A does not receive traffic from Switch B on the same port. UDLD detects the problem and disables the port.
Figure 12-1 Unidirectional Link
UDLD Aggressive Mode
UDLD aggressive mode is disabled by default. Configure UDLD aggressive mode only on point-to-point links between network devices that support UDLD aggressive mode. With UDLD aggressive mode enabled, when a port on a bidirectional link that has a UDLD neighbor relationship established stops receiving UDLD packets, UDLD tries to reestablish the connection with the neighbor. After eight failed retries, the port is disabled.
To prevent spanning tree loops, nonaggressive UDLD with the default interval of 15 seconds is fast enough to shut down a unidirectional link before a blocking port transitions to the forwarding state (with default spanning tree parameters).
When you enable UDLD aggressive mode, you receive additional benefits in the following situations:
- One side of a link has a port stuck (both Tx and Rx)
- One side of a link remains up while the other side of the link has gone down
In these cases, UDLD aggressive mode disables one of the ports on the link, which prevents traffic from being discarding.
Note In UDLD normal mode, when a unidirectional error is detected, the port is not disabled. In UDLD aggressive mode, when a unidirectional error is detected, the port is disabled.
Release 15.0(1)SY1 and later releases support fast UDLD.
Fast UDLD is a per-port configuration option that supports UDLD message time intervals between 200 and 1000 milliseconds. Fast UDLD can be configured to provide subsecond unidirectional link detection. (Without fast UDLD, the message time intervals are 7 through 90 seconds).
When configuring fast UDLD, note the following guidelines and restrictions:
- Fast UDLD is disabled by default.
- Normal and aggressive mode both support fast UDLD.
- Fast UDLD ports do not support the link debounce command.
- Fast UDLD supports only point-to-point links between network devices that support fast UDLD.
- Configure fast UDLD on at least two links between each connected network device. Fast UDLD does not support single-link connections to neighbor devices.
- Fast UDLD does not report a unidirectional link if the same error occurs simultaneously on more than one link to the same neighbor device.
- Fast UDLD cannot detect unidirectional links when the CPU utilization exceeds 60 percent.
- Fast UDLD is supported on 60 ports with a Supervisor Engine 2T.