[Openstack-operators] Need some help to understand neutron

James Denton james.denton at rackspace.com
Tue Sep 1 22:28:55 UTC 2015

Hi Salman,

The external bridge, br-ex, does not require an IP address. When routers are connected to the bridge, it is true they procure an address from the external provider network. The address is applied to the qg interface within the router namespace.

When instances send traffic out, the router NATs the traffic using its respective IP (known as SNAT, or many to one.) When an instance has a floating IP, the router will NAT the traffic using the floating IP instead (known as one to one). The SNAT behavior can be disabled on a particular router if necessary, but it introduces routing issues that need to be solved upstream.


Sent from my iPhone

On Sep 1, 2015, at 4:33 PM, Salman Toor <salman.toor at it.uu.se<mailto:salman.toor at it.uu.se>> wrote:


Can some one please explain it a bit or redirect me to the document.

So on neutron node we need external interface br-ex (public IP) and than each tenant's router also occupies a real IP when we attach it to external net.

I am wondering what is the path instances traffic take to go out? I mean will it go via the public IP attached to the router or its via br-ex?


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