Layer 2 LOOP

Hi all,

Does anyone know how a native Vlan mismatch in Cisco switch creates bridging loops?

 

Thank you for your Help

 

Regards

 

Nabs

Comments

  • Native vlan carries administrative protocols like cdp, vtp and stp.

    if you send spanning-tree in the navitve vlan 1 but the peer switch sends you spanning-tree bpdus in vlan 2 you wont expect those bpdus to come in that vlan

    and then a bloking port could transition to forwarding creating the loop.

     

    HTH

     

    Santiago E

     

  • I tried labbing this up and could not confirm my theory... so I removed it to avoid confusion :-)

  • Hi Santiago and Nikola,

     

    Thank you very much for your reply.

    I have 2 Distrib Switches that connect Access Switch and sometimes the administrator of access switches do bad configuration of native vlan and It do a loop . So my question is: Do you know if is it possible to avoid this ?

     

    Nabs

  • Take a look at this enhancements of spanning tree.

    HTH

    Santiago E




    http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_50_se/configuration/guide/swstpopt.html







    On Fri, May 29, 2009 at 9:49 AM, Nabs <[email protected]> wrote:

    Hi Santiago and Nikola,

     

    Thank you very much for your reply.

    I have 2 Distrib Switches that connect Access Switch and sometimes the administrator of access switches do bad configuration of native vlan and It do a loop . So my question is: Do you know if is it possible to avoid this ?

     

    Nabs



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    CCNA/CCDA/CCNP
  • Hi,

    I know all of these technics but they don't talk about how to avoid mismatch native vlan.

     

    Nabs

  • As much as the problem is no longer theoretical but practical :), can you post some more info on the topology?

    Are you positive the cause of the loop is a native vlan mismatch on the other side?

    Have a look at this if you haven't already:

    http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml

















  • If
    you use ISL trunk you wont have this problem because isl doesn’t implement the
    concept of native vlan

     

     

    :ccie[email protected]] En nombre de nikola

    Enviado el:
    Viernes, 29 de Mayo de
    2009 14:44

    Para: [email protected]

    Asunto: Re: [CCIE R&S] Layer 2
    LOOP

     

    As much
    as the problem is no longer theoretical but practical :), can you post some
    more info on the topology?

    Are you
    positive the cause of the loop is a native vlan mismatch on the other side?

    Have a
    look at this if you haven't already:

    http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml







    Internetwork Expert - The Industry Leader in CCIE Preparation

    http://www.internetworkexpert.com



    Subscription information may be found at:

    http://www.ieoc.com/forums/ForumSubscriptions.aspx





  • Wouldn't tagging the native in dot1q have the same effect?

  • I only want to understand the theory of how a mismatch native vlan can cause a loop. But it's not my problem.

    I have a real loop problem and I expose it to you:

     

    spanning-tree on vlan 1 is not activate on SW1, SW2, SW3 and SW4.

     

    SW1---------Trunk vlan 500--------------SW2

    |                                                         |

    Trunk vlan 500                                Trunk vlan 500

    |                                                         |

    |                                                         |

    SW3--------trunk vlan 100,500----------SW4

    Fa0/1(access vlan 100)               Fa0/1(access vlan 100)                                                 

    |                                                         |

    |                                                         |

    |                                                         |

    |                                                         |

    SW5--------------------------------------SW6

    The fa0/1 interface of sw3 and sw4 is in access vlan 100.

    When I plug SW5 and SW6 with default cisco configuration and servers on them, i have a loop on sw1, sw2, sw3 and sw4. (I wait at about 2 hours and the CPU climb to 100 %)

    I don't understant why???

    When I configure the correct sw5 and sw6 with spanning-tree in vlan 100 (and trunk between sw5 and sw6) there is no problem.

     

     

     

     

     

  • Hi all,

    I have find something about mismacth native vlan on the BCMSN cisco book:

    page 2-92 "if there is a native VLAN mismatch on either side of an 802.1q link, Layer 2 loop may occur because VLAN 1 STP BPDUs are sent to the IEEE STP mac address 0180.c200.0000 untagged."

     

    But I haven' find anything yet for my real problem.

     

    Nabs

  • Hey all,

    I have find the problem with a default stp configuration that can make a loop layer 2:

    The problem is on duplex mismatch configuration. Look at this:

     

    http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml#duplex

     

    So becareful with cisco default configuration and your particular switch configuration.

    Regards

    Nabs

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