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        ISP1                                ISP2
           |                                 |
Modem/Router1 <---------------------------Modem/Router2
                                       |    |  |   |   |
                                      PC1 pc2 pc3 pc4 pc5

Router1: 192.168.1.1 255.255.255.0 DHCP Server

Router2: 192.168.1.2 255.255.255.0 DHCP: None

PC5: 192.168.1.50 (Statically set in Router 1) 255.255.255.0 192.168.1.2

I need help. I'm trying to setup a dual ISP setup @ my home network.

Setup above currently works for a few minutes, then for some reason the mac/address listed for Modem2 changes and becomes un-accessible directly. The network still works, yes. But I can no longer use ISP2.

The Idea is that all machines use ISP1 for their network, except for 1 special machine PC5.

When I use ARP -A on any client machines, I notice that the mac address assigned to 192.168.1.2 changes to 5c-b9-01-41-b5-ac, a device that doesn't exist in our home anywhere!

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  • 1
    Your network setup is valid, unless there is a piece your not stating... something else is going on your not seeing. 5CB901 is an HP device, to be honest, this sounds like an IP address conflict with 192.168.1.2. I assume you have edited the DHCP settings of router 1 so that 192.168.1.2 is not part of the DHCP pool?
    – acejavelin
    Mar 13, 2016 at 14:35
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    Lackenzie C. Brito MAC addresses don't pass through routers, the MAC address is coming from inside your LAN not from the Internet. Again, the MAC address changing is a tell tale sign of an IP address conflict.
    – acejavelin
    Mar 13, 2016 at 15:20
  • 1
    Not a god, just happened to spot the real issue... I will post a properly formatted answer, please mark it as acceptable if it answers your question.
    – acejavelin
    Mar 13, 2016 at 15:35
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    As a general tip, when doing anything at all complex, it's best to use a slightly weird subnet to make sure you're not conflicting with any other device. Everyone uses 192.168.0.0/24 or 192.168.1.0/24; try using 192.168.47.0/24.
    – Mike Scott
    Mar 13, 2016 at 16:17
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    @MikeScott As a general rule, never use 192.168.0.0/24 or 192.168.1.0/24 for anything beyond basic setup, the defaults of almost every device fall in these ranges.
    – acejavelin
    Mar 13, 2016 at 16:21

2 Answers 2

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Your network setup is completely valid, as long as only one device is responding to DHCP requests then a network can have two (or more) routers with no issues, but to utilize anything but the default gateway you must statically assign it to the nodes or build routing statements, but that doesn't seem to be the issue here, it appears to be more basic than that.

If the network is functioning, then suddenly your ARP table changes and the MAC address associated with 192.168.1.2 is changing, this is a tell tale sign of IP Address conflict, meaning more than one device is trying to use 192.168.1.2. Looking at the stated MAC address of 5c-b9-01-41-b5-ac which seems to be "stealing" this IP address on a MAC Address lookup site shows that 5CB901 is associated with a Hewlett Packard device. It appears that some HP device is trying to use 192.168.1.2 causing the router at that address to be unusable.

Find the HP device that is trying to use 192.168.1.2 and remove it from the network or change it so the device will not use that IP address and you should be back in business.

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  • Thank you. When I was setting up my initial network topology I was totally unaware that my HP work laptop had it's own IP address as 192.168.1.2. Also, thank you for the MAC Address lookup site, that would be very helpful for me in the future.
    – Luigi
    Mar 13, 2016 at 16:49
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The first problem is that you probably have two conflicting DHCP servers which will be fighting. Right off the bat you need to disable both routers' DHCP servers and set one of the computer to dish out addresses. It can selectively tell specific computers to use specific gateway/DNS settings.

Secondly, a possibly better approach is to get a gateway that supports load balancing, such as this TP-Link Load-Balancing Router.

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2
  • This network setup is completely valid, assuming there isn't more than is stated. There is no reason one router can't be the DHCP server if the other one is disabled.
    – acejavelin
    Mar 13, 2016 at 14:33
  • Yes. In my settings only Router 1 is acting as DHCP server. But for some reason when I'm arping, the mac address assigned to the 192.168.1.2 changes to an unidentified macaddress, during this time. I am also no longer able to access 192.168.1.2 (my router 2) and it's mac address is no longer found anywhere in my ARP requests. It vanished.
    – Luigi
    Mar 13, 2016 at 14:35

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