the doc below says do not use peer switch in a mixed envrionment
Peer-switch feature is supported on networks that use vPC and STP-based redundancy is not supported. If the vPC peer-link fail in a hybrid peer-switch configuration, you can lose traffic. In this scenario, the vPC peers use the same STP root ID as well same bridge ID. The access switch traffic is split in two with half going to the first vPC peer and the other half to the second vPC peer. With the peer link failed, there is no impact on north/south traffic but east-west traffic will be lost (black-holed).
This sets the bridge ID on both switches to the system ID for EVERY vlan including the non VPC ones. Now as long as you set a priority the non VPC swtiches at least work
But on the interswitch routing link, where you often forget to set a root bridge (after all it is only point to point), you get the exact same bridge IDs and the on side of the vlan goes Back BLK which means that loopguard fired.
removed peer-switch and all was well
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