Port Management
Loopback Detection
Cisco 220 Series Smart Switches Administration Guide Release 1.1.0.x 112
9
• 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 SYLOG message.
Configuring Loopback Detection
Default Settings and Configuration
Loopback detection is not enabled by default.
Detection interval is 30 seconds.
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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