If a port configured with STP loop guard stops receiving BPDUs, the port will be put into which
state?
A.
Learning state
B.
Listening state
C.
Forwarding state
D.
Loop-inconsistent state
Explanation:
STP Loop Guard
Feature Description
The STP loop guard feature provides additional protection against Layer 2 forwarding loops (STP
loops). An STP loop is created when an STP blocking port in a redundant topology erroneously
transitions to the forwarding state. This usually happens because one of the ports of a physically
redundant topology (not necessarily the STP blocking port) no longer receives STP BPDUs. In its
operation, STP relies on continuous reception or transmission of BPDUs based on the port role.
The designated port transmits BPDUs, and the non-designated port receives BPDUs.
When one of the ports in a physically redundant topology no longer receives BPDUs, the STP
conceives that the topology is loop free. Eventually, the blocking port from the alternate or backup
port becomes designated and moves to a forwarding state. This situation creates a loop.
The loop guard feature makes additional checks. If BPDUs are not received on a non-designated
port, and loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state,
instead of the listening / learning / forwarding state. Without the loop guard feature, the port
assumes the designated port role. The port moves to the STP forwarding state and creates a loop.
When the loop guard blocks an inconsistent port, this message is logged:
CatOS%SPANTREE-2-LOOPGUARDBLOCK: No BPDUs were received on port 3/2 in vlan 3.
Moved to loop-inconsistent state.
Cisco IOS%SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port FastEthernet0/24 on
VLAN0050.
Once the BPDU is received on a port in a loop-inconsistent STP state, the port transitions into
another STP state. According to the received BPDU, this means that the recovery is automatic
and intervention is not necessary. After recovery, this message is logged:
CatOS%SPANTREE-2-LOOPGUARDUNBLOCK: port 3/2 restored in vlan 3.
Cisco IOS%SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port
FastEthernet0/24 on VLAN0050.http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094640.shtml