# Combining two broadband connections



## h8red69 (Sep 16, 2005)

Hi,

I was wondering if it is possible to combine two broadband connections as follows:

The existing connection is coming through the phone line into a router which is connected to an integrated NIC in the computer. Is it possible to add another NIC to the PC and run another broadband connection into it? The other broadband connection would be a wireless connection to an ariel outside with a lan cable running straight into the new NIC. This is Windows XP..

The addressing would be like this:
First connection router has DHCP its address is 192.168.1.254
So the original LAN connection probably gets an address of 192.168.1.1
Gateway and DNS servers are automatically picked up.

The second connection would be on a WAN with static addressing given by the provider.
The IP address, subnet mask, gateway and DNS servers would all be quite different to
the addresses on the first connection.

I'm not sure how clear I've made myself!

Anyone have any idea what I'm talking about or if something like this is possible?

Thanks.


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## O111111O (Aug 27, 2005)

Well, there are a few different solutions. The problem that you're going to have is acheiving any kind of symmetric load balancing, and keeping track of load balancing. I can tell you, your bandwidth won't be 1+1. Any given TCP conversation will only travel one source/destination path pair. So if you're kicking off an FTP, and it travels your Broadband - it will only consume your broadband interface.

You're going to need a router/firewall that allows you to do interface based stateful routes and NAT's. Checkpoint KINDA does this. IPtables/Linux will KINDA do this. What you end up having to do many times is insert CIDR specific routes pointing out a specific gateway (i.e. send my WWW traffic out my Broadband, but my SMTP in/out my dedicated)

Also, keep in mind that inbound traffic (i.e. if you have a webserver/email/etc) will probably be forced to traverse the dedicated connection. You'll have issues with assymetric routes/pathmtu/reverse path verification/all kinds of headaches.

One hardware solution I've tested in the lab, and the only one that I've seen that does a decent job is made by Radware. It's the LinkProof. Makes this a bit easier. Your other option is two dedicated connections and BGP.

http://www.radware.com/content/products/lp/default.asp


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## h8red69 (Sep 16, 2005)

Thanks O111111O

The reason I ask is moreso to have a kind of backup going where if the primary service goes down that the other one would cut in. If I didn't set up any type of routing for specific traffic would all traffic go through LAN1 and if that went down would everything then go through LAN2? Probably not that simple? If not I won't worry too much about it. I just had a friend who asked would this be possible as his connection tends to go down quite a lot..


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## JohnWill (Oct 19, 2002)

What about a simple Dual-WAN router? They will load balance, and if one WAN link goes down, it uses the other one.


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## O111111O (Aug 27, 2005)

Yeah, dual WAN connected to the same ISP wouldn't be too hard. Floating static from the ISP if you have dedicated IP would more or less round-robin source-destination TCP conversations, although items such like CEF would typically make one connection used much heavier than the other. 

Doesn't help the situation if you have disparate ISP's, different types of connections, etc. Again, if you were to plan on having dual connections with the same static route with the same weight, a routing table will have a hard time determining where a source packet came frame, and ensuring that the TCP conversation went out the same interface. (Notice, keep saying TCP. Non stateful connections aren't as bad.)

If you're looking for something to use just for backup, that's a little bit easier. Is this outbound, I assume?

Two weighted statics on some form of router can handle that to a certain extent. The issue is Ethernet based connections.  Typically a static route will only drop out of a routing table if a link goes down. If the router in question that owns the routing table is connected Ethernet to both devices (Wireless and Broadband) - and the Ethernet link stays UP but maybe the network connection (i.e. Broadband cable modem/DSL) is down, then the static route will probably stay in there.

Sorry to ramble. 

You can attempt this with two weighted default gateways. You'll need something a bit stronger than a Linksys type "router". Now that I'm thinking about this, Windows in general, or Windows/ISA server may provide SOME assistance. However you will most likely find that you'll have to do some form of manual intervention in the event that one link fails.

Example:
Interface 1 Broadband. 192.168.1.2 255.255.255.0
Gateway 1 Broadband. route 0.0.0.0 0.0.0.0 192.168.1.1 1
Interface 2 Wireless. 10.0.0.2 255.255.255.0
Gateway 2 Wireless. route 0.0.0.0 0.0.0.0 10.0.0.1 100
Interface 3 Yournetwork 172.16.0.1 255.255.255.0
Gateway 3: nothing.

Your systems use default GW of 172.16.0.1. If it's Windows/ISA server (or Linux/IPtables) then you can have DNS/BIND running as cache mode forwarder. It will use both of your ISP's DNS servers round-robin fashion (hint, disable NSCD in Linux or DNS Client in Winders. This will allow your ISA/Iptables firewall and your PC's to always resolve current DNS names in the event that there's a failure)

So. In my long winded example above; your firewall will always use 192.168.1.2 as it's default gateway for outgoing traffic. If the link for 192.168.1.2 is removed, the route will be removed and the next route in line (10.0.0.1) will take over.

Again, it'll work ok with outbound/backup. Your biggest difficulty will be your router actually determining when one of it's links are down. With Linux you can do this with script to determine next hop availability to summarily remove route. With windows, I'm sure there's some snazzy package that does the same.

Again, sorry for long winded reply. Lots of coffee today..


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## JohnWill (Oct 19, 2002)

A simple dual-WAN router on different ISP's works well, I've seen several of them at various clients. I have no idea how "perfect" the load sharing is, but it's clear that both links are active all the time, and when one is lost, the other one services the entire load. The beauty of the solution is in it's simplicity, at the LAN end of the router, you don't have to do anything special.


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## O111111O (Aug 27, 2005)

For outbound access, it will work. If you're counting on it for load sharing, stateful inbound access, or "keeping up" your access without intervention in the event that one ISP connection goes down.. Not so much.


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## JohnWill (Oct 19, 2002)

If you're planning on keeping a specific socket connection intact when one of the lines goes down, then I'd agree. OTOH, if you're saying that you can't continue to use the Internet after a connection goes down, then you'd be incorrect.


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## O111111O (Aug 27, 2005)

John.

How do you think a device adds or subtracts a route from it's route table? Usually by link, correct?

If you have two ethernet adapters in your windows system. Put two default routes out, do a route print. Now, disconnect Ethernet on one of those adapters. Do another route print.

-------

If you have two links via Ethernet to a cable modem, DSL modem, or wireless device, and the DSL side of your network goes down. How will the device in question determine that the gateway is now dead?


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## JohnWill (Oct 19, 2002)

I think you're missing the point. The router is handling the two ISP lines, your machines only require a single NIC. I've seen this work for a long time in a couple of different locations, and neither client has had any complaints about service when one ISP link goes down.


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