Wired Intelligent Edge (Campus Switching and Routing)

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Occasional Contributor I

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

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!

 

Occasional Contributor I

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

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!

Occasional Contributor I

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

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?

MVP Expert

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

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

 






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PowerArubaCX: Powershell Module to use ArubaCX API (get interface/vlan/ports info)



ACMP 6.4 / ACMX #107 / ACCP 6.5
New Contributor

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

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.

Highlighted
MVP Expert

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

A good explanation could definitely be found here:

 

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

 

 

 

 

Occasional Contributor I

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

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! 

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