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Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

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  • 1.  Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

    Posted Oct 24, 2018 04:48 PM

    I've noticed that we have some switches with trunks (LAG) that automatically enable spanning tree and set a priority of 4 on the switch. This happens regardless of the spanning tree configuration or if spanning tree is disabled or enabled. 

     

    Can someone explain the theory behind this? I've used MSTP on Brocade and Dell switching, including LAGs, but have never set a different priority for the LAGs specifically. 

     

    Best practice to leave these priorities in, change them to the instance priority or...?

     

    Also, from my understanding I should put the VLANs that we use into a new instance and leave the ist as it's own. So I'm setting both the instance 1 and the ist priority the same. Is this correct? Previously I've always just set the priority and never moved the configured VLANs in a separate instance. I can see times where we could use the multiple instances to spread traffic across different interfaces though and that is why I moved our active VLANs to instance 1 instead.

     

    Thanks!

     



  • 2.  RE: Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

    Posted Apr 04, 2019 10:12 PM

    It seems no one knows why trunks automatically get spanning tree priority of 4, so I'm bumping this up to see if anyone has an answer for why or suggestions on how it plays in to the overall spanning tree operation. Thanks!



  • 3.  RE: Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

    Posted Apr 05, 2019 11:07 AM

    I've received 4 kudos for posting again to this thread so it seems I'm not the only one wondering this. Is there an @aruba SE or something that could jump in here with an answer?



  • 4.  RE: Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

    MVP GURU
    Posted Apr 05, 2019 12:06 PM

    i think it is historic... and it is for priority (force ?) a "Trunk/LAG" comparing the other link...

     



  • 5.  RE: Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

    Posted Apr 05, 2019 04:51 PM

    I believe the idea is, say you have switch1 and connect downstream to switch 2.  If you connect a trunk (LAG) as your primary downlink between the two, it gets a priority of 4 automatically.  If you (purposly or accidently) plug in an extra cable between the two switches, you don't want it to use port priority (lower number interface wins) and potentially move your active root port and designated port down to the just plugged in cable, but rather keep it on the LAG that you assumedly have more bandwidth or at least have configured for the uplink.



  • 6.  RE: Trunk/LAG gets spanning tree priority 4 regardless of MSTP config
    Best Answer

    MVP GURU
    Posted Apr 06, 2019 04:12 PM

    A good explanation could definitely be found here:

     

    https://support.hpe.com/hpsc/doc/public/display?docId=emr_na-c02950043

     

     

     

     



  • 7.  RE: Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

    Posted Apr 10, 2019 11:11 AM

    Thank you to all of you for replying. I apologize I did not reply earlier as I have been out sick.

     

    Now this makes sense, and how I assumed it worked. The link helped immensely in explaining it, I had searched and searched but must have been using the wrong keywords in Google to find that!

     

    So basically, the priority on the trk LAGs is to prioritize the LAGs in case you have more than one, or like another answer above, you would connect another cable and/or LAG to the same set of switches. You can prioritize which LAG is the primary, secondary, etc. By default it gets priority 4, but that # can be modified to set the priority of each trk# LAG.

     

    Also, this priority would NOT come into play for the switch priority, and does not modify the full switch's priority in the STP configuration of the network, just the priority of the trks / LAGs on the switch itself.

     

    Thank you so much! 



  • 8.  RE: Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

    Posted 25 days ago
    Apologies for reopening this, but i noticed the same priority settings applied to a trunk i have on a 2530. However in my own case the LAG is connected to a QNAP storage not another switch. I wonder if this will cause any STP issues. Is it ok to go ahead and remove this spanning tree setting from the trunk since it is not a LAG to another switch?


  • 9.  RE: Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

    MVP GURU
    Posted 24 days ago
    Hi, you are wondering if "this will cause any STP issues", it shouldn't.

    It should work flawlessly also with that setting (my experience), below a Trk1(with four 1 Gbps copper links) to a QNAP NAS:

                        |           Prio              | Designated        Hello
      Port   Type       | Cost      rity State        | Bridge            Time PtP Edge
      ------ ---------- + --------- ---- ------------ + ----------------- ---- --- ----
      Trk1              | 20000     64   Forwarding   | xxxxxx-xxxxxx     2    Yes Yes


    with this configuration:

    interface Trk1
       untagged vlan 1000
       spanning-tree admin-edge-port
       spanning-tree priority 4
    exit
    


    and each port has this configuration:

    interface 1/A24
       name "QNAP-NAS-Port-1"
       untagged vlan 1000
       trunk trk1 lacp
    exit
    
    interface 1/B24
       name "QNAP-NAS-Port-2"
       untagged vlan 1000
       trunk trk1 lacp
    exit
    
    interface 2/A24
       name "QNAP-NAS-Port-3"
       untagged vlan 1000
       trunk trk1 lacp
    exit
    
    interface 2/B24
       name "QNAP-NAS-Port-4"
       untagged vlan 1000
       trunk trk1 lacp
    exit


    with this LACP status:

               LACP      Trunk     Port                LACP      Admin   Oper
       Port    Enabled   Group     Status    Partner   Status    Key     Key
       -----   -------   -------   -------   -------   -------   ------  ------
       1/A24   Active    Trk1      Up        Yes       Success   0       983
       1/B24   Active    Trk1      Up        Yes       Success   0       983
       2/A24   Active    Trk1      Up        Yes       Success   0       983
       2/B24   Active    Trk1      Up        Yes       Success   0       983