Switching

 View Only
last person joined: 3 days ago 

Ask questions and share experiences about EX and QFX portfolios and all switching solutions across your data center, campus, and branch locations.
  • 1.  spanning-tree interface priority question

    Posted 03-28-2021 15:35
    From my understanding, if the interface cost is equal, the interface priority is used to breaker. the higher, the better.  I am testing to make sure my understanding is correct. But seems not, I do not know what I missed here.
    lab@ex2# run show spanning-tree interface                             
    
    Spanning tree interface parameters for instance 0
    
    Interface                  Port ID    Designated         Designated         Port    State  Role
                                           port ID           bridge ID          Cost
    ae0                          128:3        128:3   8192.c042d009bda1        10000    FWD    ROOT 
    ge-0/0/3                   128:490      128:490   8192.c042d009bda1        20000    BLK    ALT  
    ge-0/0/4                   128:491      128:491   8192.c042d009bda1        20000    BLK    ALT  
    
    Spanning tree interface parameters for instance 1
    
    Interface                  Port ID    Designated         Designated         Port    State  Role
                                           port ID           bridge ID          Cost
    ae0                          128:3        128:3   4097.c042d009bda1        10000    BLK    ALT  
    ge-0/0/3                   128:490      128:490   4097.c042d009bda1         9999    FWD    ROOT 
    ge-0/0/4                   240:491      128:491   4097.c042d009bda1         9999    BLK    ALT  
    
    Spanning tree interface parameters for instance 2
    
    Interface                  Port ID    Designated         Designated         Port    State  Role
                                           port ID           bridge ID          Cost
    ae0                          128:3        128:3   4098.c042d009be61        10000    FWD    DESG 
    ge-0/0/3                   128:490      128:490   4098.c042d009be61        20000    FWD    DESG 
    ge-0/0/4                   128:491      128:491   4098.c042d009be61        20000    FWD    DESG ​

    From the above, I expected to see ge-0/0/4 elected to be ROOT as it has a higher priority, but it is not, why ?

    thanks for insights !!



  • 2.  RE: spanning-tree interface priority question

    Posted 03-28-2021 16:47
    No, lowest port priority is preferred.  Also, you look at your neighbor's port priority, not your own.  Check our my article about STP: 

    https://momcanfixanything.com/spanning-tree-protocol-summary/

    Regards, 


  • 3.  RE: spanning-tree interface priority question

    Posted 03-28-2021 17:20
    Thanks !!
    I will take a look .


  • 4.  RE: spanning-tree interface priority question

    Posted 03-28-2021 17:30
    I have two switches with 3 interfaces connected, ae0, ge-0/0/3 and ge-0/0/4.
    lab@ex2# run show spanning-tree interface 
    
    Spanning tree interface parameters for instance 0
    
    Interface                  Port ID    Designated         Designated         Port    State  Role
                                           port ID           bridge ID          Cost
    ae0                          128:3        128:3   8192.c042d009bda1        10000    FWD    ROOT 
    ge-0/0/3                   128:490      128:490   8192.c042d009bda1        20000    BLK    ALT  
    ge-0/0/4                   128:491      128:491   8192.c042d009bda1        20000    BLK    ALT  
    
    Spanning tree interface parameters for instance 1
    
    Interface                  Port ID    Designated         Designated         Port    State  Role
                                           port ID           bridge ID          Cost
    ae0                          128:3        128:3   4097.c042d009bda1        10000    BLK    ALT  
    ge-0/0/3                   128:490      128:490   4097.c042d009bda1         9999    FWD    ROOT 
    ge-0/0/4                    64:491      128:491   4097.c042d009bda1         9999    BLK    ALT  
    
    Spanning tree interface parameters for instance 2
    
    Interface                  Port ID    Designated         Designated         Port    State  Role
                                           port ID           bridge ID          Cost
    ae0                          128:3        128:3   4098.c042d009be61        10000    FWD    DESG 
    ge-0/0/3                   128:490      128:490   4098.c042d009be61        20000    FWD    DESG 
    ge-0/0/4                   128:491      128:491   4098.c042d009be61        20000    FWD    DESG ​

    After I lowed the priority on ge-0/0/4 on MSTI 1, ge-0/0/4 should be ROOT port ?

    thanks !!




  • 5.  RE: spanning-tree interface priority question
    Best Answer

    Posted 03-28-2021 17:36
    If you are changing the priority on EX2, nothing is going to change on EX2.  As I said, you need to look at your neighbor's priority,  not your own. 

    If you want EX2 to chose ge-0/0/4, change the priority of the interface on EX1


    Regards, 


  • 6.  RE: spanning-tree interface priority question

    Posted 03-28-2021 17:41
    thanks a lot !!   I am to give a try.


  • 7.  RE: spanning-tree interface priority question

    Posted 03-28-2021 21:25
    thanks so much for your clear explanation. I just did in the lab and it worked as you explained.