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Matthew Rankin
Asked: 2009-11-20 18:25:43 +0800 CST

Same or separate subnets when connecting two buildings via a wireless bridge

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We have two buildings about 200 feet apart connected via SMC's outdoor wireless bridge. Currently, both buildings are on the same subnet with one DHCP server in the main building. This seemed to be the easiest way to setup our network so that clients in either building could have access to the Samba drive in the main building. However, one problem with this configuration is that if the wireless bridge link goes down, then the remote building no longer has a DHCP server.

So I'm wondering if it would be better to have the two buildings setup on separate subnets. If I were to convert the two buildings to separate subnets, what configuration changes would I need to make in order for the clients in the remote building be able to have access to the Samba drive in the main building?

Other, Potentially Pertinent Configuration Details

  • Linksys RV082 Wired Router in the main building provides DHCP as well as VPN access for remote clients
  • Each building has a Linksys WRT54G Wireless Router with DHCP disabled to provide wireless Ethernet in each building
  • Aside from the Ubuntu server providing the Samba drive, we also have two other servers in the main building both running Ubuntu
  • The majority of clients are WinXP Pro laptops; however, we do have a few Ubuntu and a few Mac OS X clients
  • All servers and network connected printers are configured with static IPs via the RV082. All clients are configured for DHCP.
dhcp

3 Answers

  1. Personally, I'd set them up as separate, if only to keep the broadcast traffic from going over the wireless.

    I think that once you had set up the subnets, the only change required would be to make sure that the samba server accepted the other subnet as a valid client source.

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  2. Best Answer

    If you are only concerned about DHCP availability, then don't bother with a separate subnet. There shouldn't be any problem at all setting up 2 DHCP servers on the same subnet as long as they are handing out different ranges of IP addresses. It looks like your existing hardware should support it and it would solve the primary problem you mentioned. It also has the added benefit of some redundancy in case one of the routers stops handing out addresses, which seems common with little appliances like these. If you continue to run one subnet, remember that your machines will pick up addresses from either DHCP server.

    There are other benefits to separate subnets such as limiting your broadcast traffic and the ability to use a routing protocol to provide redundant links in the future. Before adding this complexity, I would figure out whether limiting broadcast traffic will provide a noticeable performance improvement, either now or as your network grows. If you are going to grow significantly in the future, it might be better to just break your subnet up now rather than having to readdress a huge chunk of your network down the road.

    If you do go with separate subnets, you will have a different gateway for the machines on the second subnet and obviously all of the routing to get between subnets. I also expect you would need to set up and use the WINS server that comes with SAMBA. (SMB doesn't play nice on separate subnets without WINS)

    And as a quick aside, I have Cisco and RAD bridges linking several fire stations up over distances of several miles, and I can't say enough about good antennas. You might find your bridges to be a lot more reliable if you can get some dish or yagi antennas.

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  3. I haven't looked up the specs on your bridge, but in general a bridge is a layer 2 device like a switch, so it's not subnetting you need to look at.

    The only concern you mention is lack of a DHCP server if the bridge is down, but unless it goes down a lot and for a long time, the clients on the other side will still have their addresses until their leases run out. If the leases are expiring during an outage, it might be enough to simply make the lifetime longer. I'm not familiar with the RV082, so you're on your own for that.

    Another option would be to have a DHCP server on the "other" side of the bridge, but exactly how you do that will also depend on the RV082. I haven't used a WRT54G for a while, but here's a sample of a WRT configuration page. You can configure it to hand out addresses starting at a certain IP, so if the RV082 can be configured to hand out IPs below that, you can have DHCP servers on either side of the bridge that hand out different ranges of addresses.

    This answer is the good explanation of what a bridge is (and the best answer to that question.)

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