Loop Protect Port Modes

Ports work in two Loop Protect operational modes.

In full mode, when RSTP/MSTP BPDUs is received in point-to-point link and the port is designated, a Loop Protect timer is set to 3 times hello time, when this timer expires then port will be moved to blocking state. Limited mode adds a further requirement that the flags field in the BPDU indicates a root role.

Message age expiration and the expiration of the Loop Protect timer are both events for which Loop Protect generates traps and a debug message. In addition, user can configure Loop Protect to forcefully disable port when one or more events occur. When the configured number of events happens within a given window of time, the port will be forced into disable and held there until you manually unlock it.

The following example shows the loop due to the misconfiguration in STP:

Switch 1 Elected as Root shows that Switch 1 is elected as Root. Switch 2 and Switch 3 elect the root port. Switch3‘s port connected to Switch2 is elected as Alternate port and it is port state is in blocking state.
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Switch 1 Elected as Root
STP Disabled on Switch 2 shows that if the user accidentally disables the STP on Switch 2 port connected to Switch 3, Switch 2 will stop sending the BDPU to Switch 3 since STP is disabled. Switch 3 assumes that neighbor is down and it changes the port to forwarding state which will eventually create a loop.
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STP Disabled on Switch 2
Loop Protect Enabled shows that, with loop protect enabled switch 3 will not go to forwarding state until it receives a BPDU from switch 2 and the port state will be in discarding state.
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Loop Protect Enabled
When the Loop protect feature is enabled:

You can enable the port by giving the command enable port port-list.