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Combining two DSL connections for LAN connectivity

Posted on 2002-05-07
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Last Modified: 2013-11-30
Hello,

I would like to have redundant DSL connections to my office, and be able to have both be active concurrently (transparent to my users, of course).  Hopefully, this will make my office connectivity less subject to the vagaries of a single DSL provider.

However, my existing DSL providers gives me static ip addresses, while the potential new providers only offer dynamic ip addresses.

What network configuration would make this work (e.g., dual port router with NAT on one of the ports, etc.)?

Additonal background:
1. My clients all have static ips and will retain those
2. Currently with single DSL connection I have a pretty simple setup as follows:

     dsl line <-> dsl router <-> firewall <-> switch <-> lan ... clients


Is there any other info needed?

-RD
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Question by:rdefuria
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11 Comments
 
LVL 8

Accepted Solution

by:
scraig84 earned 400 total points
ID: 6994833
Since you have static IP's my assumption is that you are hosting something from this site.  If you intend to keep those IP's active once the "primary" dies, you are trying to squeeze blood from a turnip.  DSL is merely a step above dialup and is not considered a "serious" technology to anyone who truly wants reliability and fault tolerance in a hosting scenario.  There is no DSL provider that I have ever heard of that will allow customers to run BGP at their sites.  I have never even heard of getting multiple lines from the same provider and allowing the ISP to perform the re-routing upon line failure.  You may be able to call every ISP in the books and find one, but it is doubtful.  I would also question the sanity of the provider that would actually provide this service.

However, if your intention is to keep the casual web surfer active once one of the lines goes down, a router could be placed behind the two DSL modems and/or existing routers and have equal cost static routes.  That would be fairly simple from a load sharing standpoint.  You would still have a problem once one of them went down (half of the outbound packets would be lost).  There isn't much you can do about that.  

I would recommend that you move to another (more spendy) technology that is built to handle failures appropriately.  Basically, it comes down to getting what you pay for.

I hope that helps!
0
 

Author Comment

by:rdefuria
ID: 6994865

Heavens, no, we are not hosting anything from our office; all our production stuff is located at our data center (ostensibly much more robust than anything we could do internally).

My intention is just what you said - to keep office connectivity (web browsing and sending/receiving email, mainly) when our primary DSL goes down.

I was hoping for some slick solution that would work whether one or the other DSL providers was down.  If one went down, couldn't the cost of routes be adjusted so that no outgoing packets get lost?  I guess I am not following what you suggest...

-RD
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LVL 8

Expert Comment

by:scraig84
ID: 6994998
Here is the problem.  DSL "modems" usually connect to the inside device via an Ethernet connection.  Even if the DSL link goes away, the Ethernet side stays up and keeps a link with the connected router or PC.  Therefore even if you have a router there, it has no way of ever determining if the route went down, because the link literally never dies.  So, it always thinks it has an active link and will never choose a secondary path.  If you want simple load balancing, a router can decide to share both by considering both links to be equal.  However, if one goes down, it still sends half of the packets to the side that's down.  Unless some DSL modems have a feature to bring down the Ethernet interface in the event that the DSL connection goes down (which I've never heard of), you aren't really left with any options.
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LVL 41

Expert Comment

by:stevenlewis
ID: 6995312
I have to agree with scraig84, you can't get there from here
0
 
LVL 56

Expert Comment

by:andyalder
ID: 6998641
Radware LinkProof in between the LAN and 2 DSL routers ought to work. Doesn't need BGP, it just tries both routes and choses the faster one.
http://www.radware.com/content/products/link.asp
0
 
LVL 56

Expert Comment

by:andyalder
ID: 6998671
Oops, missed that you needed "receiving email". That may not be possible if you have a SMTP server recieving email.

You'd have to have 2 MX records, one resolving to your permanent IP address and the other (with higher number/lower preference) resolving to the dynamic IP supplying ISP's mail relay which can forward to your dynamic IP address through their mailserver knowing what IP address you have through their radius servers logs but they may not offer this service.
0
 

Author Comment

by:rdefuria
ID: 6998705

We are not accepting incoming smtp connections in the office.  That's done by our mail server in a data center.  We use pop-3 to read our email, and make outgoing smtp connections to send email.  That's what I meant by sending/receiving email (I should have more correctly said reading/sending email).

So will this device allow for one of my dsl providers giving me static ips, and the other giving me dynamic ips?

What happens if returning packets come back via the non-NATed path?

Will it recognize if one dsl connection is down and route all traffic the other way?

I am not sure I have a handle on whether this is really possible or not, or whether
I will have to split up my LAN and have some folks use one gateway and the rest use another gateway, and manually reconfigure if one of the providers goes down.


0
 
LVL 56

Expert Comment

by:andyalder
ID: 6998968
POP3 makes life easy.
The box itself does NAT so the packet will have the IP address of one of it's interfaces so the reply will come back to the same interface. Pretty sure it can handle mixture of static and dynamic ISP provided addresses. If one path is down (even if it's the ISP's backbone rather than the local DSL connection) it will notice this and use the other interface. Guess you might loose current sessions that were going through whichever ISP fails.
 I'm not a radware salesman but the box will do exactly what you want. Splitting the LAN by having half of clients with one gateway and half with the other doesn't solve the problem of www.foo1.com being faster through ISP1 and www.foo2.com being faster through ISP2 but this box does.
0
 

Author Comment

by:rdefuria
ID: 6999322

I am looking for a more cost-effective solution.  Would a linux box to do routing, NAT, firewalling, bridging, etc. be helpful in this scenario?

0
 
LVL 56

Expert Comment

by:andyalder
ID: 6999766
No cost-effective solution. scraig84 said it all in his first comment, I just added one of the products available as the thread was still open 2 days later. Don't give him less than an A just because it can't be done for free.
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Author Comment

by:rdefuria
ID: 7001214

Thanks, everyone, for all the input and expertise!
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