I have cases open with both Cisco and Juniper on this, but wanted to see if anyone else had seen an issue like this because support has no idea. I have a Juniper QFX 5100 Core running in Virtual Chassis mode with 4 switches. I have 4 separate stacks of Cisco 3750 switches with 2x1GB uplinks bound into 4 different LACP trunks. I have had it happen twice now where I apply a trunk port config(not an LACP trunk) to a port that isn't a part of any of the LACP trunks and it causes all 4 of the Etherchannels on the Cisco stacked switches to go into an err-disable state with these messages: Mar 14 07:11:33: %PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Gi1/0/48, putting Gi1/0/48 in err-disable state Mar 14 07:11:33: %PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Po17, putting Gi1/0/48 in err-disable state Mar 14 07:11:33: %PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Po17, putting Po17 in err-disable state Mar 14 07:11:34: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/48, changed state to down Mar 14 07:11:33: %PM-4-ERR_DISABLE: channel-misconfig (STP) error detected on Gi2/0/48, putting Gi2/0/48 in err-disable state (CA-TOR-1-7-2) Mar 14 07:11:34: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet2/0/48, changed state to down Mar 14 07:11:34: %LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel17, changed state to down Here is the config I am applying to the port that has caused this issue to happen twice now: set interfaces ge-0/0/67 description "Firewall Port" set interfaces ge-0/0/67 unit 0 family ethernet-switching interface-mode trunk set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 9-10 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 29 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 31-32 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 43 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 50-51 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 56 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 58 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 66 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 68 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 90 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 143 set interfaces ge-0/0/67 unit 0 family ethernet-switching vlan members 170 The issue happens within a couple of minutes of committing the config on the Juniper side, there are no cables plugged into port 0/0/67 so technically there shouldn't be any BPDU's sent out since there isn't a port change. Juniper Support wants me to turn on trace option and then run though a bunch of scenarios, the issue is that testing this takes down my network. Just wanted to put it out there to see if anyone else had run into a situation similar to this. TIA Joe