[Openstack-operators] Need some help to understand neutron

Salman Toor salman.toor at it.uu.se
Wed Sep 2 12:40:29 UTC 2015

Thanks for the response and also for the link :-)

Actually we are struggling with the floating IP so each one of them matters.

I am thinking to go for a model where I can use minimum floating IPs, can’t afford to give a new floating IP to each new tenant's routers.

 Since its not a production environment we can compromise on performance and different tenants traffic can share same interface.


PhD, Scientific Computing
Researcher, IT Department,
Uppsala University.
Cloud Application Expert,
Visiting Researcher,
Helsinki Institute of Physics (HIP).
salman.toor at it.uu.se<mailto:salman.toor at it.uu.se>

On 02 Sep 2015, at 00:28, James Denton <james.denton at rackspace.com<mailto:james.denton at rackspace.com>> wrote:

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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