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[Openstack] [neutron][tripleo] Neutron external bridge setting

Hello everyone,

Could someone kindly help explain the following paragraphs taken from
the OpenStack TripleO documentation [1]?

    "By default, Neutron is configured with an empty string for the
Neutron external bridge mapping. This results in the physical
interface being patched to br-int, rather than using br-ex directly
(as in previous versions). This model allows for multiple floating IP
networks, using either VLANs or multiple physical connections.

    When using only one floating IP network on the native VLAN of a
bridge, then you can optionally set the Neutron external bridge to
e.g. â??br-exâ??. This results in the packets only having to traverse one
bridge (instead of two), and may result in slightly lower CPU when
passing traffic over the floating IP network."

Here I have difficulties to understand the traffic flow in the case of
setting an empty string for the Neutron external bridge mapping
(NeutronExternalNetworkBridge). My interpretation is that an empty
string would map a physical interface directly to the br-int instead
of the br-ex. Does it mean to bypass the br-ex and use the br-int for
external traffics? If so, how come this would be less efficient as
oppose to using br-int <-> br-ex <-> physical interface? This is where
I got confused. Any helps would be greatly appreciated.

Thank you very much to all.