Open Stack

> neutron net-create public --tenant_id a0edd2a531bb41e6b17e0fd644bfd494  --provider:network_type flat --provider:physical_network default --shared True
> Invalid input for provider:physical_network. Reason: '[u'default', u'True']' is not a valid string.
> 
> For being able to use --provider:physical_network I need bridge_mappings in configuration, right? When I add it, my existing GRE network stops working.
> It seems I am lost here ...

You should be able to run bridge-mapped networks and GRE tunnels at the same time. 
Adding the bridge map config should not break GRE. (always do this in a test setup first ;)
We used to do this up to Folsom (maybe even grizzly, do not remember exact timelines)

We moved to a full VLAN setup later on because GRE was adding complexity without any real benefits. 
(Since we do not expect to have thousands of networks we do not expect to run out of VLANs)

Cheers,
Robert van Leeuwen