Does a PC with 2 NICs on different networks cause a loopback or performance problem?

Posted on 2007-09-28
Last Modified: 2012-05-05
Would a machine configured with two NICs, each configured for a different network but plugged into the same switch via 2 separate cables cause a loop problem or any other network performance problem?  This diagram may help illustrate what I mean:
Question by:themodusoperandi
    LVL 23

    Accepted Solution

    If the nics are configured for separate subnets then no. are they set up on separate vlans on the switch? If not then only one nic will work (the one that is configured for the same subnet as the switch)

    Im not sure why you would want to do this though. Not a good design idea
    LVL 9

    Assisted Solution

    do you have routing between those NICs enabled on the PC? if yes what are the routes?

    Author Comment

    Definitely not a design idea.  This was a machine that was put in our network before I started this job by another company who we send information to through a VPN.  Really its just acting as a router.

    Here's how a little bit more detailed diagram of what it's being used for:

    Our LAN is
    We send a raw image to the 2 NIC machine from a diagnostic machine.  The images are compressed using a proprietary program, then are transported to the remote site via A VPN Tunnel on the subnet.  All switches on this network are unmanaged so VLANs have not been manually configured.  

    @avilov - could you explain to me where to check to see if routing is enabled in XP?  
    If youre talking about HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\IPEnableRouter
    That is currently set to 0

    Honestly we're not experiencing a problem with this particular setup, but we are having users in other areas that are noticing intermittent network performance issues, so I just wanted to rule this out before I continue troubleshooting.  
    LVL 79

    Assisted Solution

    The preferred way to handle this is definately with VLAN's, but if you have unmanged switches, they may not support vlans.
    Typically, separate Layer3 networks over the same Layer2 switch infrastructure is not a big problem, except for broadcasts which get seen by both networks. This is why we recommend VLAN's to create broadcast boundaries.
    It "shouldn't" be causing any issues, but if you are experiencing network issues, it is certainly one thing to consider. Without a managed switch infrastucture it is really difficult to troubleshoot.

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