Comment 6 for bug 1689683

Reading this bug I think I'm missing something on what Juju expects from Neutron AZ's in terms of choosing networks etc.

AFAIK the AZ implementation in Neutron is designed to allow network services for a particular resource to be spread across underlying physical/network/power failure domains - so for example you could create a network:

 $ neutron net-create --availability-zone-hint AZ1 \
      --availability-zone-hint AZ2 new_network

which would schedule the various network namespaces and daemons (dnsmasq, metadata proxies etc) across agents residing in zones AZ1 and AZ2. Likewise went a router is created, to provide north/south network traffic flows from the tenant network to the outside world, its created as a HA router with zone hinting to multiple availability zones:

 $ neutron router-create --availability-zone-hint AZ1 \
      --availability-zone-hint AZ2 --ha new_router

I think that reading:

"21:23:49 ERROR juju.cmd.juju.commands bootstrap.go:491 failed to bootstrap model: cannot start bootstrap instance: cannot allocate a public IP as needed: could not find an external network in availablity zone"

Juju is expecting to find an external network in the same AZ as the instance being created - but that does not sound sensible to me - the instance availability zone locates and instance to a specific failure domain in the DC, whereas the network availability zone allows a network or routers resources to be spread across underlying failure domains.