Error while creating attached to an external interface

I'm trying to launch an instance attached to an external network, but I get an all host exhausted error message, and the neutron log file show the following error : *ERROR neutron.plugins.ml2.managers [req-6979d5fd-8e6a-4802-a758-e4882b7ac5b6 397fa0267ae340e1b24a3a96ae302f32 05b9f31fca434f7a96b63a1d17e8b14c - default default] Failed to bind port d57ac8f5-7904-495f-a4b9-9b23df681634 on host compute1.localdomain for vnic_type normal using segments [{'id': '475c70c2-6a35-4721-a726-cbf8c0b7e778', 'network_type': 'flat', 'physical_network': 'physnet2', 'segmentation_id': None, 'network_id': 'a1c2cc8f-6a04-48ce-9d87-a351da1d3f3e'}]* But when I try to launch an instance using another external network, it works just fine.