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Loop Guard Questions

November 19th, 2019 in SWITCH 300-115 Go to comments

Question 1

Question 2


Loop guard feature provides additional protection against 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.

Question 3


Loop guard prevents alternate and root ports from becoming designated ports, and spanning tree does not send BPDUs on root or alternate ports.

Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_55_se/configuration/guide/3560_scg/swstpopt.pdf

Note: Loop guard prevents root ports and alternative ports from moving to forwarding state when they stop receiving BPDUs.

  1. Burìk
    November 1st, 2019


    Answer A doesn’t sound right to me. Loop Guard does not detect or isolate unidirectional link failures, it prevents loops *caused by* unidirectional link failures.

    With version 7.1(1) of the Catalyst software (CatOS), loop guard can be enabled globally on all ports. Effectively, loop guard is enabled on all point-to-point links. <– D is correct.
    The loop guard works on non-designated ports and does not allow the port to become designated through the expiration of max_age <– E is correct


  2. Brian
    November 4th, 2019

    I believe Burik is correct.

  3. King in the castle
    January 21st, 2020

    I agree with others. The correct answers should be D & E.

  4. John2020
    February 14th, 2020


    Please confirm Q3 , which is the right answer?


  5. Tester
    February 16th, 2020

    Actually answer A,D

    It does prevent unidirectional link failure same as udld, E is wrong because it prevents backup (not root port) from going into designated port

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