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I've seen some questions that were similar to this, such as:

Multiple access points for the same SSID?

But I'm dealing with a bit of a special case, in that the internet provider for this apartment building has two modems, one on each side of the building. Each of the modems has a router right beside it, and one of those routers has an ethernet cable connected to another router in the center of the building.

Apparently wireless clients are smart enough to pick the closest router for a given SSID, so I've set the routers to the same SSID, on channels 1, 6, and 11. My concern is how DHCP should work. I'm thinking that if both of the modem-adjacent routers have the same IP, and DHCP enabled, then someone who gets a lease on the west side of the building and then walks over to the east side should be able to use that lease? I suppose the question here is whether there's anything to the DHCP lease that will prevent it working on another network with the same configuration?

Any other concerns with this setup?

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3 Answers

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For the two routers that are connected, set up dhcp on the router connected directly to the modem. The other router should disable dhcp completely, and plug the cable into a LAN port instead of the WAN port. This will allow the two devices to work together better.

The remaining router that is attached to the second modem should be given a different SSID. You don't want this router on the same network as the others, because it has a different gateway.

If you really want this to all be one network, you need to run a network wire from one modem towards the other, and have a gateway device that can make use of both modems at once. This device could be a linux box with the right software set up and three network cards (one for each modem and the first router). You will also need to run a network wire from the third router to first, and configure the third router as you did the 2nd (disable dhcp, connect the wire to a LAN port rather than the WAN port).

At this point, you'll likely want some kind of edge device for monitoring and defending your network. Where I'm at, we use untangle. However, to do what you want will require the WAN Balancer app, which depending on the size of the building will likely run you about $270/yr. You probably also need bandwidth control and policy manager, to prevent a user running bittorrent in one apartment from draining all the bandwidth from everyone else, but that can get expensive.

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I was afraid that would be your answer. I'm dealing with very non-technical users who I have limited-to-no communication with, so my preference would be a single SSID, even if it means a few dropped packets when they hop between gateways. The ISP offers a plan that has 2x the speed of what either modem is getting, so probably I could just use a single modem and the three routers. I don't want to run any cables, but we don't always get what we want. –  Alec Munro Sep 21 '11 at 18:30
    
Unfortunately, they won't just hop between gateways. Just name the SSID's "MyApartment1" and "MyApartment2". The connection software on people's machines will do well enough helping users pick the one with the stronger signal. –  Joel Coehoorn Sep 21 '11 at 18:31
    
This looks like a good candidate to trunk the modems together: cisco.com/en/US/products/ps9924/index.html Runs about $400, but it should be a solid piece of a equipment. –  Joel Coehoorn Sep 21 '11 at 18:33
    
Joel is right. Having the names the same caused serious problems. :( –  Alec Munro Sep 22 '11 at 17:20
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I see no reason why you shouldn't be able to have the same SSID on all of the wireless routers. As long as they have the same SSID and same Key your users should be able to move trough the building without even realizaing that they are switching between wireless access points. The gateway and ip information will change when they switch between routers but as long as they don't have a static address the software in their OS should handle changing gateway and IP address information.

This is what I suggest: For the modem that you have two routers connected to; get an inexpensive ethernet switch, connect the modem to the switch instead of directly to the first wireless router, then plug both wireless routers into the switch as well. Turn on DHCP for both routers and set them up with the same SSID and security key. For the single modem/router on the other end of the building, set it up for DHCP with the same SSID and security key that you gave the other two routers. Then take a laptop and walk from one end of the building to the other testing the connection every few feet or so, you should hop routers without really even noticing. Hope this helps!

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Also, you shoulden't really have to get so high-tech or expensive on the security scheme that you use. Most Wireless routers have built in firewalls that you can use for protection, in this situation all of your users will have to be responsible for ensuring their own security, such as installing anti-malware programs. If you really want to go into more indepth security, get a REALLY cheap desktop like from the paper or on ebay and install Ubuntu desktop and install wireshark and some additional free security utilitys such as IDS/IPS utilites, just search google for ideas. –  Dan Sep 21 '11 at 19:03
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Yes, you should be able to assign the same SSID to all of your wireless access points (WAPs) and clients should be able to roam between them.

HOWEVER, when doing this kind of setup, the clients will try their best to stay on the same AP. If and when they roam to the next one, the client will actually disconnect momentarily and re-lease it's IP from DHCP. So if you were streaming a movie, then roam to another WAP, the movie will stop playing.

In a true multi-WAP setup (where the WAPs are all acting in concert, not merely configured the same way), there will be no disconnection when roaming. This is due to the way the system will broadcast itself and because, usually, these enterprise systems will tunnel traffic from clients back to the firewall/controller.

You should also be careful with co-channel interference from WAPs that are too close together. As you know, channels 1-6-11 are non-overlapping channels... so if there are two APs on the same channel that can "hear" each other, you will take a performance/reliability hit. Especially on a network such as yours where all the WAPs are not part of a fully-integrated system.

Regarding your DHCP environment, I don't feel that I interpreted your setup 100% correctly, but there should be only one DHCP server in the same Layer-3 address space. I understand that you have two modems, so I ASSUME you DO have multiple DHCP sources. However, since you seem to be running independent WAPs, this shouldn't be a problem... as long as they can't see each other. Now, your clients might get confused as they roam from DHCP domain to DHCP domain... And of course, users will only ever have access to one of the internet connections (modems) at a time.

A better solution would be to go with a fully-integrated solution with mesh WAPs that can all talk to each other and act as "over-the-air" routers. This way, you could have one DHCP server for the entire setup, and traffic can be automatically routed to the best internet connection (modem). These kinds of networks are also self-healing, which means that if a WAP goes down, the remaining WAPs can route around it. This redundant, meshed solution will also allow users to SEAMLESSLY roam from WAP to WAP without disconnects.

A fully-integrated system such as I'm recommending would also allow you much more control over how things work, who can get connected, configure multiple wireless SSIDs on the same WAPs, configure VLANs, Layer-2/3 ACLs, captive portal, more secure (and easier!) authentication mechanisms, and much more.

Full disclosure: My company (CWB) is your Ruckus Wireless solutions provider in the Middle Tennessee area.

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I guess I shouldn't assume that you are near Nashville! You guys could be anywhere! haha –  Brent Reynolds Sep 28 '11 at 20:54
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