One Subnet - Multiple VLANS?

[craigus1988]

Hi,

Is it possible to have multiple VLANS that all share the same IP range and all share a common gateway address?

The reason I ask is for the purposes of live migration (V-MOTION) where the IP information of the migrated host needs to remains unchanged (to not break any sessions with clients). Migrating to one physical host in the same VLAN is easy, but I am trying to see if it can be done across VLANs(hence the need for the common IP information) I am aware of VXLAN and have herd the term LISP passed around a few time but I am looking to see if it is possible with commodity hardware, as I don't have any fancy high end kit to play around with 

I have been looking at things such as "IP unnumbered" to try and assign each vlan gateway a common address but I am not seeming to get any joy out of this.

Can anyone shed any light on this?

[Richard Burts]

Craig

There are several problems that you will encounter if you try to do this.

- if it is the same IP subnet across several VLANs then each device will think that another device (to which it wants to communicate) is in the same subnet and will ARP for the destination MAC address. But the ARP request is a local broadcast and will not get forwarded to the other VLAN and so the ARP will fail. And if ARP fails then IP communication will fail.

- if you have multiple VLANs then I assume that each VLAN would have a interface vlan x. But if you create multiple VLAN interfaces and put IP addresses in the same subnet on them then I would expecct that IOS would complain about overlapping addresses and not accept the second interface IP address.

The only way that I can think that would make this work would be to bridge all the VLANs together (probably by connecting an access port in one VLAN to an access port in another VLAN so that you get one big broadcast domain. This would allow one subnet to exist on multiple VLANs but you have defeated the reason that you used VLANs in the first place which was to provide separation.

I do not see a good solution to make your migration work.

HTH

Rick