Dell Force10 C150 Configuration Manual page 1065

As shown in STP topology 3 (Figure 52-12 bottom middle), after you enable loop guard on an STP port or
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port-channel on Switch C, if no BPDUs are received and the timer expires, the port transitions
from a blocked state to a loop-inconsistent state (instead of to a forwarding state). Loop guard blocks the
STP port so that no traffic is transmitted and no loop is created.
As soon as a BPDU is received on an STP port in a loop-inconsistent state, the port returns to a blocking
state. If you disable STP loop guard on a port in a loop-inconsistent state, the port transitions to an STP
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blocking state and restarts the timer.
Spanning Tree Protocol | 1065