Multiple ADSL's

Unanswered Question
Feb 21st, 2007

Hi Guys, I would like to bounce the following off of you and hear your views.

We have a client who is desperate for more external BW. They have requested an increase in BW on their serial circuit but this could take a while.

An idea I have (after reading other posts on here is as follows).

The network is very messy, they have 3 different ADSL connections from 3 different providers, servicing three different departments (we can ?borrow? these ADSL?s for the short term. Plus the majority of users using the 512 connection.

They are really really desperate for more BW than their current 512kbps serial connection.

Is it possible to combine the 3 ADSL connections back to one router and have 3 default routes pointing out of the 3 dialer interfaces controlling the 3 ADSL connections.

All users are on one class C private network and the plan will be with route maps to run overload on each ADSL interface.

I'm trying some different things on some spare routers we have here but I wondered if anyone has actually configured this in the real world.

Will it work ????

Thanks in advance.

I have this problem too.
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lgijssel Wed, 02/21/2007 - 02:43

It will not work as you hope. Using three different external nat adresses will garble your sessions. You could try to implement per-user load balancing by dividing the workstations in groups (ip ranges) and redirect each group to a different gateway using policy routing based on their source adresses.

This will never realize an equal division of bandwidth between the users but at least you can use what's available.



graces Wed, 02/21/2007 - 02:56

Thanks for your input Leo. If it's not too much hassle could you expand a bit when you mention the sessions getting garbled?

I was thinking also along the lines of policy routing, that will be something else for me to look into.

As I mentioned their network really is a mess, we are looking for a short term fix as a) We hope to get the increase of the leased line (fingers crossed) and b) rumour control says they should be moving to new premisses within the year..although you know how these things work out.


lgijssel Wed, 02/21/2007 - 03:10

When packets are load balanced, this normally occurs on a per-packet basis. This would cause packets that belong to the same session to be distributed over three paths (routers).

Each path has a different private ip adress that it uses to nat the traffic. In this scenario, although the destination ip is the same, it will not be possible for the receiving host to re-group all packets that belong to the session.

The link below will help you to realize a setup using policy routing:



graces Wed, 02/21/2007 - 03:27

Hi Again, Yes you are correct but I wondered if CEF was configured whether this would solve this problem being load balenced per destination. Sorry if I'm missing something, it's a strange idea and something a bit different from the norm so I don't have any practical experience to back up the theory.

Thanks Simon.

lgijssel Wed, 02/21/2007 - 03:40

Using CEF might solve this issue partly but to be honest, I did not consider that.

In my opinion, what your customer needs is to load-balance either per session or per source.

Using policy routing is a reliable way to implement per source load-balancing.



graces Wed, 02/21/2007 - 05:47

It would be interesting to see how CEF behaves, although until you try in the real World it's difficult to imagine how things will behave.

But you are right, I think policy routing will be less of a head ache. Thanks again Leo.

rlowton Wed, 02/21/2007 - 13:00


I'd have thought using GLBP to create a virtual IP for all three routers would be the way to go here. This way you have a common subnet between the 3 departments and stick the three ADSL routers into that common pool with the GLBP address being the default gateway for the PC's. The internal hosts will be 'load balanced' roughly across the three providers.

The BIG issue i think for this configuration is making sure that if one of the ADSL links goes down that you can still work properly (ie. if one links goes down then 1 in 3 lookups will no longer work). So you'll probably be also adding some object tracking to the GLBP config.



graces Wed, 02/21/2007 - 23:26

Hi Rich, I must admit, this isn't something I've seen before. I'll have to have a read up and see if it will help with our issue.

Thanks, Simon.


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