Wired Intelligent Edge

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!

 

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!

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?

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

 

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.

A good explanation could definitely be found here:

 

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

 

 

 

 

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! 

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?
Original Message:
Sent: Apr 06, 2019 04:11 PM
From: Davide Poletto
Subject: 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

 

 

 

 

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

Original Message:
Sent: Nov 08, 2022 09:19 PM
From: Kola Oladapo
Subject: Trunk/LAG gets spanning tree priority 4 regardless of MSTP config

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?
Original Message:
Sent: Apr 06, 2019 04:11 PM
From: Davide Poletto
Subject: 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