Configuring multiple default gateways on DHCP server

Posted on 2011-10-26
Last Modified: 2012-05-12
I have DHCP running on my Windows Server 2008 R2 domain controller.  It is currently handing out a default gateway, which is my local router.  I have another site, connected to my main site by a T1 connection, that I would like to add to my domain.  They have their own internet connection (router); they only connect to the main site to access an IBM server running our financial system.  The remote site is on the same subnet as the main site (mainly to eliminate the need for another server), and I would like take them off of static addressing and put them on DHCP.  The issue I am foreseeing is that I do not want to send remote internet traffic through the small T1 line to the router in the main building.  I know there is a way to configure additional gateways on a DHCP server and order them by preference.  How does the server/client decide which one to use?  Is it just by availability, or is it judged perhaps by a metric?  Point is, I would like to send DHCP through the T1 line, but have internet traffic stay local.  Is there a way to configure this or am I better off installing a remote DHCP server?
Question by:Dustin23
    LVL 17

    Accepted Solution

    When a computer has multiple interfaces and multiple "default routes" it will always use the interface with the lowest Metric.  If two interfaces have the same metric, then it will use the first interface in the list.  When one interface has mutliple default routes, it will use them in the order that the DHCP server provided them, and will always try to use the top one in the list first.  You cannot tell "clients" to pick the "closest" (fastest) one

    Unfortunately, your plan is flawed - for several reasons.

    The "remote site" being on the same subnet as the "local site" means you've got a single broadcast domain, as you've effectively got a stretched LAN.  This is ill-advised, and you will run into all sorts of complications, including being unable to route between the two sites, being unable to prioritize which site routes to where, and more importantly, assuming the two "segments" of the network can talk to each other - any broadcasts which occur will be echoed across the T1 line.

    You're best off making the remote site a completely different subnet, and then you can create a new zone for that subnet on your DHCP Server (which is over the link) - but you will somehow have to tell those clients to use this DHCP server.  You would do that by setting up a Helper address on the router on the remote site, or by setting up a DHCP Relay on a computer at the remote site.  Since it will be a different subnet, you are going to have to set up routing between the two networks in some fashion.

    Author Closing Comment

    @ LesterClayton

    Ok, thanks for the input.  Looks like I will be setting up another domain controller at my remote site...

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