[openstack-dev] [Neutron]Relationship between physical networks and segment

Robert Kukura kukura at noironetworks.com
Tue Mar 29 21:47:45 UTC 2016

My answers below are from the perspective of normal (non-routed) 
networks implemented in ML2. The support for routed networks should 
build on this without breaking it.

On 3/29/16 3:38 PM, Miguel Lavalle wrote:
> Hi,
> I am writing a patchset to build a mapping between hosts and network 
> segments. The goal of this mapping is to be able to say whether a host 
> has access to a given network segment. I am building this mapping 
> assuming that if a host A has a bridges mapping containing 'physnet 1' 
> and a segment has 'physnet 1' in its 'physical_network' attribute, 
> then the host has access to that segment.
> 1) Is this assumption correct? Looking at method 
> check_segment_for_agent in 
> http://git.openstack.org/cgit/openstack/neutron/tree/neutron/plugins/ml2/drivers/mech_agent.py#n180 
> seems to me to suggest that my assumption is correct?
This is true for certain agent-based mechanism drivers, but cannot be 
assumed to be the case for all mechanism drivers (even all those that 
use agents. Any use of mapping info (i.e. from agents_db or elsewhere) 
is specific to an individual mechanism driver. I'd strongly recommend 
that any functionality trying to make decisions based on connectivity do 
so by calling into the registered mechanism drivers, so they can decide 
whether whatever they manage has connectivity.

Also note that connectivity may involve hierarchical port binding, in 
which case you really need to try to bind a port to determine if you 
have connectivity. I'm not suggesting that there is a requirement to mix 
HPB and routed networks, but please try not to build assumptions into 
ML2 plugin code that don't work with HPB or that are only valid for a 
subset of mechanism drivers.
> 2) Furthermore, when a segment is mapped to a physical network, is 
> there a one to one relationship between segments and physical nets?
Certainly different virtual networks can map to different segments (i.e. 
VLANs) on the same physical network. It is even possible for the same 
virtual network to have multiple segments on the same physical network.

> Thanks
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