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I was wondering if it is possible to build a network that allows two subnets to be linked by wireless routers with identical network SSIDs and security protocols in such a way that a DHCP Client can switch between the two without losing any packets.

I've heard that once upon a time there was an idea for IPv4 mobility, allowing cross-subnet roaming by relaying things through a host network, but supposedly it never caught on.

Is it possible for one router (access point), to detect when a client is disconnecting using DHCP Release, and then forward any further traffic for that client out to other access points to get relayed by whichever AP has the client connected to it?

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    You may want to look at wireless controllers if that is a requirement. If using a single controller, all packets can exit the controller's interface so roaming from one AP to another does not have to change your vlan or subnet. They also support intra-subnet roaming (i.e. cisco.com/c/en/us/td/docs/wireless/controller/7-5/config_guide/…) – some_guy_long_gone Jul 24 '14 at 13:07
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer. – Ron Maupin Jan 6 at 2:29
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Not with different subnets, no. You can use the same subnet, same VLAN, and same SSID on both APs and you'll be able to roam between them. If you try to use a new subnet, the client will need a new IP address or you'll risk duplicate IPs when they roam, plus you'd need some crazy NAT or VRF setup to talk from one side to another.

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Short answer: Soon!

Longer answer: There is an enhancement to TCP called "MPTCP" which will help you accomplish this kind of thing. Kernels are starting to support it. Check out http://queue.acm.org/detail.cfm?id=2591369 for an overview.

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  • Can you add some detail on MPTCP? I tried looking into it but the RFC did not give me any information I could digest. – OneChillDude Jul 25 '14 at 17:19
  • There are projects to add this featurre to both Linux multipath-tcp.org and FreeBSD caia.swin.edu.au/urp/newtcp/mptcp. I'm told that the iPhone has this already, but since most websites don't support it on their end you can't take advantage of it yet. – TomOnTime Jul 26 '14 at 12:11
  • Hmmm, so it's not client independent? – OneChillDude Jul 26 '14 at 15:53
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One thing that you could do is configure wifi roaming between the two APs with network AB (subnet A + subnet B) but in the first AP there's a DHCP server only for network A and in the second AP the DHCP server for network B. This way, a client would have an IP address from its network of origin and still be able to roam.

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There are vendor-specific solutions that might get you what you want, even though it may not be exactly what you asked for. You create one SSID for the entire building. You get an IP address that is associated with that wifi base station. From then on, as you move around the building (or campus) you keep that IP address. The network just "does the right thing" for you. I believe http://www.arubanetworks.com/ is one company that has this feature; there are likely many others.

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