VIRL topology - STP blocking loop on VLAN 1 only

[Anthony L]

Not sure if this is a quirk with VIRL or my limited understanding with STP. 

My setup:

• Two trunk links exist between switches SW1 and SW2 and are not in an etherchannel
• VLANs 1, 10 and 20 are defined on both switches and active
• Both switches have default priority values for all VLANs. Due to SW1 having the lowest numerical base MAC address, it becomes the root bridge for all VLANs with all ports assuming the "Designated" port role and forwarding status (as expected)
• Running show spanning-tree on SW2 shows STP is blocking one of the two trunk link interfaces for VLAN 1 (Gi0/2) to prevent a loop - which I'd expect - however, not expected is that it is keeping both interfaces in the forwarding state for VLAN 10 and 20 - i.e. not prevent a loop on those VLANs.

Please can someone help me understand why STP on SW2 is not blocking one of its two trunk links for VLAN 10 and VLAN 20 and if there is a way to make it block one?

I've uploaded the VIRL file. 

[Martin L]

Yes, I have seen this behavior in VIRL before. There are several issues posted in CCIE RS Study group in CLN. If you go there, search posts in that group about virl, you should see at least 3 recent (this year) posts regarding Virl layer 2 image.
I would not use VIRL to study switching; I would use it to do routing.

I think your "issue" is same as spotted by Steven in https://learningnetwork.cisco.com/thread/133540

https://learningnetwork.cisco.com/groups/ccie-routing-and-switching-study-group

there is also a VRIL group where u can posts Qs about VIRL and get responses from VIRL people.