Weird spanning tree problem

[tbsanjose]

I recently joined a new company where they use ICX7450 as core and distribution switches.

Distribution switch is connected by trunk to 6 3rd party field switches. 

Additionally these field switches have the following connections.

1 and 2 connected together

3 and 4 connected together

5 and 6 connected together

All field switches are L2 switches so routing is not an option unfortunately.

Distribution switch has per vlan 802-1w enabled, all field switches have standard RSTP enabled.

Core switch has STP completely off (no idea why)

distribution switch has priority set to 0 on all vlans.

The problem we have:

We always need to have one port shut down for example to switch 1 or switch 2 (also either 3 or 4 and either 5 or 6) on distribution switch. When we enable both links creating a loop we get random behavior. Sometimes broadcast storm starts right away, we see distrib 7450 get CPU spike to 20% but surprisingly we had many situations where everything was working fine for hours. I logged in to field switches where I saw they correctly elected alternating route and put one port in discarding mode. No CPU spike on distribution at all. I went home only to wake up to hundreds e-mail from Solarwinds and network was completely down. Turns out for whatever reason one field switch turned it's discarding port to forwarding.

Thinking that it's a problem with classic RSTP on field switch unable to talk to per vlan RSTP on 7450 I enabled single instance of 802-1w on distrib switch. In theory everywhere except for Core we have classic RSTP. This makes problem even worse. Broadcast storm right away when we create a loop.

I have already spent 2 weeks fighting with this and I pull my hair on this. Why is it happening.

Oh I should add that our distribution switch is running SPR code because we have VE interfaces on it. Does SPR not work with STP?

Anyway please help me identify the problem.

[tbsanjose]

UPDATE:

I might have found the mistake of previous network engineer but I need confirmation.

Our default vlan is left unchanged as Vlan 1

Trunk ports are ports ethe 1/2/1 to 1/2/4, ethe 1/3/1 to 1/3/3 and ethe 1/4/1

Now I believe his mistake was that he specifically removed untagged traffic on all trunk ports

This is copy of our config:

vlan 1 name DEFAULT-VLAN by port
no untagged ethe 1/2/1 to 1/2/4 ethe 1/3/1 to 1/3/3 ethe 1/4/1
spanning-tree 802-1w
spanning-tree 802-1w priority 0

Am I right to assume that this essentially blocks BPDUs between our ICX7450 and 3rd party switches running simple RSTP because they expect untagged BPDU frames on trunk ports?

Thanks