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2807 Views 9 Replies Latest reply: Sep 5, 2011 6:44 PM by Martin RSS

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Per Vlan (R)STP

Sep 4, 2011 3:35 PM

MichaelQwerty 5 posts since
Sep 4, 2011

Hello everybody,

 

I am reading CCNA Official Exam Certification Guide and I have encountered some problems. I cannot really feel the “(R)STP instance for every VLAN” thing. IMHO the example in the book I mentioned is too easy (there are only 2 switches with the same VLAN database).

 

Let’s think about the topology given below:

 

Everywhere, there are trunk links between switches. Every switch has one VLAN1 and all interfaces in it, except:

Switch0 – additionl VLAN10 and access interface in this VLAN to PC1

Switch5 – additional VLAN10 and access interface in this VLAN to PC0

Switch5 – additional VLAN333 with no interfaces assigned

Switch1 - additional VLAN33 with no interfaces assigned

 

I have checked sh span vlan 10 on Switch0 and Switch5. Output shows that both switches are Root bridges… I thought STP for VLAN10 should be sth like this:

 

For VLAN333 sth like this:

And for VLAN1 for example sth like this:

What have I misunderstood? Please, take your time and help me.

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    1. Sep 4, 2011 5:12 PM (in response to MichaelQwerty)
    Re: Per Vlan (R)STP

    You can upload your PT file here; Attach to this post using Advanced Editor;

    Output shows that both switches are Root bridges

    One root per Vlan, as you cannot have 2 root bridges on 1 same vlan;

    Make sure all switches have the same vlans; then check which one is Root for which vlan;

     

    usually you will have to make changes manually, changing bridge priority for vlans;

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    4. Sep 5, 2011 4:36 AM (in response to MichaelQwerty)
    Re: Per Vlan (R)STP

    I could not open your file, but here is mine

     

    https://learningnetwork.cisco.com/docs/DOC-13015

     

    open it and take a look at it;  see if we talk about the same things;

     

    all switches have same vlan database;

    example 1 is with 1 sw as root for all vlans;

    example 2 is with mixed root switches per vlans

  • cadetalain 2,642 posts since
    Sep 18, 2008
    Currently Being Moderated
    6. Sep 5, 2011 7:22 AM (in response to MichaelQwerty)
    Re: Per Vlan (R)STP

    Hi,

    this shouldn't work but in PT what's stranger is that switch 2 hasn't got the mac address of the source pinging but switch 5 has it from its link to switch2.

     

    I really think PT is screwing things up but in real life this should not work.

     

    Alain.

  • Brian McGahan - 4 x CCIE, CCDE 643 posts since
    May 29, 2008
    Currently Being Moderated
    7. Sep 5, 2011 7:40 AM (in response to MichaelQwerty)
    Re: Per Vlan (R)STP

    > you cannot have 2 root bridges on 1 same vlan

    But they are. PKT file in the attachment.

     

     

    You can, but it indicates an error condition.  If two bridges that are supposed to be sharing the same VLAN both claim themselves to be the root, it means that the STP BPDUs are not being exchanged between those switches for that VLAN.

     

    For example if I claim myself to be the root for VLAN 10, and you do the same thing, it means that VLAN 10 communication is broken between us.  A common case that this occurs is that a trunk link has formed to fail somewhere in the topology, or the VLAN in question is being filtered off of a trunk link.

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    9. Sep 5, 2011 6:44 PM (in response to MichaelQwerty)
    Re: Per Vlan (R)STP
    Martin probably you have older one and that is why you cannot open my file.

    yes, another PC, another PT version, hehehehe

     

    be carefull with PT running some complex STP things; it does not work very well;

     

    There are management tools for Vlan management for Cisco and no-Cisco;

    f.e Ciscoworks or Solarwinds;

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