[Openstack-operators] Two regions and so two metadata servers sharing the same VLAN

gilles.mocellin at nuagelibre.org gilles.mocellin at nuagelibre.org
Thu Nov 26 14:49:00 UTC 2015

Hello stackers !

Sorry, I also cross-posted that question here 

But I think I can reach a wider audience here.

So here's my problem.

I'm facing an non-conventional situation. We're building a two region 
Cloud to separate a VMware backend and a KVM one. But both regions share 
the same 2 VLANs where we connect all our instances.

We don't use routers, private network, floating IPs... I've enabled 
enable_isolated_metadata, so the metadata IP is inside the dhcp 
namespace and there's a static route in the created instances to it via 
the dhcp's IP. The two DHCPs could have been a problem but we will use 
separate IP ranges, and as Neutron sets static leases with the instances 
MAC address, they should not interfere.

The question I've been asked is whether we will have network problems 
with the metadata server IP, that will exist in 2 
namepaces on 2 neutron nodes but on the same VLAN. So they will send ARP 
packets with different MAC, and will perhaps perturb access to the 
metadata URL form the instances.

Tcpdump shows nothing wrong, but I can't really test now because we 
haven't got yet the two regions. What do you think ?

Of course, the question is not about why we choose to have two regions. 
I would have chosen Host Agregates to separate VMware and KVM, but 
cinder glance should have been configure the same way. And with VMware, 
it's not so feasible.

Also, if we can, we will try to have separate networks for each regions, 
but it involves a lot of bureaucracy here...

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