Remote Fault Detection

Remote fault detection is a proprietary feature that prevents a loss of traffic if a physical or signaling error occurs on a switch-to-switch fiber link.

A remote fault is an error that one switch can detect but the other switch cannot. For example, if a transmit fiber breaks, the remote port continues to receive data and so detects that the link is good. However, the remote port cannot detect that the data it transmits is not received by the local port.

If remote fault detection is enabled and a remote fault occurs, the local switch sends a message to inform the remote switch of the fault. The remote switch then shuts down the remote port.

If the receive signal is restored on the local port, the local port sends a message to the remote switch, which then turns on the remote port again.

Important: Remote fault detection must be enabled on both the local port and remote port for the feature to work.

For example, in Figure�65:

1. Port 1 on Switch A is not receiving traffic from Port 2 on Switch B.

This problem could occur because:

• The transmitter on port 2 is not functioning.
• The receiver on port 1 is not functioning.
• The fiber is broken or disconnected.
2. Switch A detects the error condition and sends a message to Switch B, which shuts down Port 2. (When the port is shut down, its status changes from Okay to No Link. The port status is displayed in the Status field of the Physical Port Configuration Web page.)

Figure�65:�Remote fault detection

Enable remote fault detection on both ends of a switch-to-switch connection, in the following two cases:

• When two Gigabit ports that do not support auto-negotiation are connected.
• When a Gigabit port that supports auto-negotiation is connected to a Gigabit port that does not support auto-negotiation.

For a list of Gigabit modules that do not support auto-negotiation, see Table�44.

Restrictions:

• Autonegotiation and remote fault detection cannot be enabled at the same time. To enable remote fault detection, autonegotiation must be disabled. When autonegotiation is enabled, remote fault detection is automatically disabled.
• If the switch is operating in Fabric mode 1, 80-series Gigabit modules support remote fault detection on only one port. If you enable remote fault detection on more than one port, a loss of traffic occurs.

If the switch is operating in Fabric mode 2, 80-series Gigabit modules support remote fault detection on two ports; however, the ports must be on different fabric ports. If you enable remote fault detection on two ports that are on the same fabric port, a loss of data occurs. For information on the relationship between fabric ports and physical ports, see "How Hunt Groups Load Share" in Chapter 6:�Using VLANs, Hunt Groups, and VTP Snooping.

50-series modules support remote fault detection on any number of ports.