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My company uses the 192.168.10.0/24 subnet and 192.168.10.235 is for all intents and purposes the gateway to the Internet. We have an extra VDSL line with a "home-style" modem/router that we want to connect as a backup. My initial idea was to connect the VDSL modem/router to a spare CISCO 878 router we have, and then connect the 878 to our switch, with an IP of 192.168.10.236, so that a change of default gateway in Windows settings is all it takes to reach the Internet via the VDSL connection, instead of our normal "enterprise" line.

The problem is that the 878 is designed to route packets from the WAN interface (which is ISDN, so of no use to us) to the 4 LAN interfaces and vice versa. The LAN interfaces are not router ports, but switch ports, all set to VLAN1. I understand, after reading official CISCO documentation, that in any 800 series router it is impossible to route from one LAN interface to another, for the aforementioned reasons.

Now, I want to ask this:

Is there a way to make this work with the equipment I have?

I now want to present an idea about a possible solution I have thought about, but please bear with me (it is a bad idea at best, moronic and unworkable at worst):

I know that I can connect the VDSL router to my switch with the 192.168.1.1 IP address, even though everything else is on the 192.168.10.0/24 subnet. If I connect the 878 on the switch, with the 192.168.10.236 IP (as per the original plan), is there a way to use a SINGLE one of its ports to route packets from 192.168.10.0/24 to 192.168.1.0/24? My idea is to be able to change the default gateway from 192.168.10.235 to 192.168.10.236 (the 878 IP address) and then have the CISCO send the packets BACK to the switch, but destined for the 192.168.1.0/24 network. Mind you my main switch is not VLAN capable. I know this is probably a long shot, and far from best practice, but I thought that maybe it could be kludged together, perhaps by setting up multiple IP addresses on VLAN1 on the 878.

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If it is OK for you to change the default gateway of your clients to fail-over, you just need to configure 192.168.10.236/24 on your VDSL router's LAN interface and connect it to your switch (note that if your are using DHCP, you should probably also disable the DHCP server in the VDSL router so you don't have two DHCP servers in the network).

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  • I am sorry, but what you are suggesting is outside of the bounds of my question.
    – dlyk1988
    Aug 7, 2015 at 9:57
  • @dsljanus maybe you are searching in the wrong direction. You have a problem to solve and I offered you a simple and straightforward solution.
    – Oliver
    Aug 7, 2015 at 10:11
  • I honestly appreciate your effort, and indeed I could have been more clear. I have considered what you offered, before coming here, and concluded that it would not be any good, because of other constraints.
    – dlyk1988
    Aug 7, 2015 at 10:13
  • @dsljanus Oliver's approach is the cheapest, easiest, and most sensible given what you have described. If there are reasons not to do it that way, please edit the question and tell us about all your "constraints".
    – Grant
    Aug 7, 2015 at 11:29

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