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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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The solution you suggest might work as long as you do not need to do NAT on the Cisco. You can simply configure eg 192.168.1.10 on the interface as follows:

FastEthernet X ip address 192.168.1.10 255.255.255.0 secondary

Then add a default route towards your VDSL router. Do not forget to also add a static route for you internal network to the VDSL router via the Cisco. If that is not possible you need to do NAT...

Another solution would be to define a second VLAN, eg. VLAN 2 on the Cisco and place one of the switchport Interfaces into that VLAN and the route between these two interfaces as above. This would also enable you do do NAT.

But the relay easiest way would be to just reconfigure your VDSL router's LAN IP to be 192.168.10.236 and disable the DHCP server on it, then connect it to your switch, magic done.

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