[openstack-dev] [Neutron][IPAM] Uniqueness of subnets within a tenant

Gabriel Bezerra gabrielb at lsd.ufcg.edu.br
Tue Mar 10 17:38:36 UTC 2015

Em 10.03.2015 14:34, Gabriel Bezerra escreveu:
> Em 10.03.2015 14:24, Carl Baldwin escreveu:
> Neutron currently does not enforce the uniqueness, or non-overlap, of
> subnet cidrs within the address scope for a single tenant.  For
> example, if a tenant chooses to use on more than one
> subnet, he or she is free to do so.  Problems will arise when trying
> to connect a router between these subnets but that is left up to the
> tenant to work out.
> In the current IPAM rework, we had decided to allow this overlap in
> the reference implementation for backward compatibility.  However,
> we've hit a snag.  It would be convenient to use the subnet cidr as
> the handle with which to refer to a previously allocated subnet when
> talking to IPAM.  If overlap is allowed, this is not possible and we
> need to come up with another identifier such as Neutron's subnet_id or
> another unique IPAM specific ID.  It could be a burden on an external
> IPAM system -- which does not allow overlap -- to work with a
> completely separate identifier for a subnet.
> I do not know of anyone using this capability (or mis-feature) of
> Neutron.  I would hope that tenants are aware of the issues with
> trying to route between subnets with overlapping address spaces and
> would avoid it.  Is this potential overlap something that we should
> really be worried about?  Could we just add the assumption that
> subnets do not overlap within a tenant's scope?
> An important thing to note is that this topic is different than
> allowing overlap of cidrs between tenants.  Neutron will continue to
> allow overlap of addresses between tenants and support the isolation
> of these address spaces.  The IPAM rework will support this.
> Carl Baldwin
> I'd vote for allowing against such restriction, but throwing an error
> in case of creating a router between the subnets.

Fixing my previous e-mail:
I'd vote against such restriction, but throwing an error in case of 
creating a router between the subnets that overlap.

> I can imagine a tenant running multiple instances of an application,
> each one with its own network that uses the same address range, to
> minimize configuration differences between them.
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