Multiple internet lines, one network

[bazianmmb]

I have a client that has both a DSL and Cable Modem. They were purchased with the intent of being able to switch between them in the event of service outages.

I would like to find out if it is possible to use them BOTH simultaneously. In effect, I would like to combine the lines and use both their bandwidth when I can. In the event of a service outage, the one working line would automatically take all the load.

Is this possible?

Thanks in advance for any information.

[mheusinger]

Hi,

this might work. The question is, how routing and load sharing looks like and also how your IP addresses. The main obstacle will be your official IP addresses and NAT requirements.

I am assuming that you use private IPs inside. Thus any IP packet leaving your network towards the internet should be NATed to the IP of the outgoing interface (cable or DSL). The return traffic will be routed back to that interface, NAT does retranslate and so far eveything is fine.

BUT if not every IP packet of a certain session is going through the SAME internet interface you will have troubles. If the first IP packet of a session is NATed to 1.1.1.1 and another packet of that session arrives at the server with 2.2.2.2 (source IP) then each server in the internet will drop it.

So your load sharing needs to make sure it is session based. IP CEF will do that per default. So you could try to use one router with two static default routes to the respective access routers or you can use one router termination both, DSL and cable.

Regards, Martin