Cisco 350 Series Administration Manual page 214

Port Management
Loopback Detection Settings
Cisco 350, 350X and 550X Series Managed Switches, Firmware Release 2.4, ver 0.4
• Direct multi-ports loop—Switch is connected to another switch with more than one
port and STP is disabled.
• LAN segment loop—Switch is connected with one or more ports to a LAN segment
that has loops.
How LBD Works
LBD protocol periodically broadcast loopback detection packets. A switch detects a loop
when it receives its own LBD packets.
The following conditions must be true for a port to be LBD active:
• LBD is globally enabled.
• LBD is enabled on the port.
• Port operational status is up.
• Port is in STP forwarding/disable state (MSTP instance forwarding state, instance 0).
LBD frames are transmitted on the highest priority queue on LBD active ports (in case of
LAGs, the LBD is transmitted on every active port member in LAG).
When a loop is detected, the switch performs the following actions:
• Sets the receiving ports or LAGs to Error Disable state.
• Issues an appropriate SNMP trap.
• Generates an appropriate SYSLOG message.
Default Settings and Configuration
Loopback detection is not enabled by default.
Interactions with Other Features
If STP is enabled on a port on which Loopback Detection is enabled, the port must be in STP
forwarding state.
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