Dell EMC SmartFabric OS10 User Guide Release 10.5.0

Far-end failure detection

Far-End Failure Detection (FEFD) is a protocol that detects remote data link errors in a network.

FEFD uses a link layer echo protocol to detect and signal far-end failures over Ethernet and optical links. When you enable FEFD, switches periodically exchange FEFD echo frames to identify link failures. If the local switch does not receive an echo from its peer for the time interval of three times the configured FEFD message interval, the local switch assumes that the peer link is down. The default interval for FEFD message interval is 15 seconds. For example, with the default configuration, if the local switch does not receive an echo message for 45 seconds from its peer, it brings the peer link down.

FEFD helps detect far-end failure when the following problems occur:

• Only one side receives packets although the physical layer (L1) of the link is up on both sides.
• Transceivers are not connected to the correct ports.

FEFD states

FEFD comprises the following four states:

• Idle—FEFD is disabled.
• Unknown—Shown when FEFD is enabled and changes to bi-directional after successful handshake with the peer. Also shown if the peer goes down in normal mode.
• bi-directional—Interface is up, connected, and receiving echoes of its neighbor.
• err-disabled—Only found when FEFD mode is aggressive and when the interface has not received three echoes of its neighbor. To reset an interface in this state, use the fefd reset command.

FEFD modes

FEFD operates in two modes—Normal mode and aggressive mode.

• Normal mode—When you enable Normal mode on an interface and a far-end failure is detected, no intervention is required to reset the interface to bring it back to an FEFD operational state.
• Aggressive mode—When you enable Aggressive mode on an interface in the same state, you must manually reset the interface.

The following events explain how FEFD state transition occurs:

• When you enable FEFD on an interface a link transitions from idle state to unknown state.
• In the unknown state, the interface starts transmitting link state information at a regular interval. The interface state changes to bi-directional when a handshake is complete with the peer.
• When an interface is in bi-directional state, if it does not receive an echo from its peer for the time interval of three times the configured FEFD message interval, the interface state changes to unknown in Normal mode. In Aggressive mode, the interface state changes to err-disabled.

If the interface state changes to err-disabled, use the fefd reset [interface] global command to reset these interfaces. The unknown or err-disabled state brings the line protocol down so that the protocols above it can detect that the peer link is down.

Restrictions

• You can enable FEFD globally or on an interface. If FEFD is enabled globally, the FEFD interface configuration overrides global FEFD configuration.
• OS10 supports FEFD only on physical interfaces. FEFD is not supported on any other interfaces. However, you can enable FEFD on individual physical interfaces that belong to a port channel.