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New Member

Load Balancing across 4 low cost DSL links


Was wondering if I could get some ideas on what would be the optimum way of loadbalancing outgoing browsing traffic across 4 outgoing DSL links that are terminated on a single router. Bear in mind that these links are capped so I need a way of removing a link from the load-balancing group once the cap on that specific link is reached.

If absolutely necessary these links could be moved onto individual routers.

Any ideas would be appreciated


Re: Load Balancing across 4 low cost DSL links


i would prefer doing policy based routing by seggregating your local lan and send the traffic belong to one group of lan on one link and ther group to another link.

By doing so your bandwidth can be effectively utilised instead of trying load balance which may not give desired results..


New Member

Re: Load Balancing across 4 low cost DSL links

Load balancing with NATted DSL connections is going to be quite difficult, if not impossible. You may be able to get it to work if you use an intermediate router with equal cost load balancing to direct outgoing connections to 4 seperate routers.

Would it not be sufficient to have 4 PPPoE connections, and fail over from one connection to the next if they go down/get capped? That configuration would be much simpler to implement.

New Member

Re: Load Balancing across 4 low cost DSL links

Thanks for the reply.

Could you please explain why it would be an issue load balacing over a nat'd DSL interfaces. What I had in mind is having all DSL connections ( 4 of them ) terminating on the same router, and then configuring 4 equal cost default routes with tracking on each of the statics. The tracking would possibly run a ping to a registered address or possibly an HTTP get, and when this fails ie Cap reached that tracked default route is removed from the routing table.

Any ideas around this ?

Thanks again

New Member

Re: Load Balancing across 4 low cost DSL links

The problem will occur because the equal cost routes will break the NAT - once a TCP connection is established out of an interface, you want it to stay on that interface. "ip route cache" may resolve that, but you may end up with very unequal load balancing.

There is another problem: your "ip nat inside source list" or "ip nat inside source route-map" statements will need to specify a dialer interface, so you may end up natting to the IP of one interface, and then sending the packet out of another. I happen to live in SA as well, and none of the ISPs allow this unfortunately.

If you had an intermediate router sending to 4 different ADSL routers, the problem above would disappear, because you'd only have a single global nat config per router. You could still set up your tracking with applicable local policy route map to send to the correct ADSL router. I don't know how happy your customer will be with 5 routers though.

I've thought of another way you could get around this - you could have one intermediate router and 1 ADSL router, and then connect the two routers together via a trunk and configure 4 subinterface. Then you could have 4 default gateways pointing from the intermediate router, and use route-maps exclusively (attched to each subinterface) to direct traffic out of the correct dialer interface.

There is a last option which may work, but is using "undocumented" features, but I've seen it work on multiple versions of IOS. It is possible to have a GRE tunnel originate and terminate on the same router. You could emulate the config above by using 4 GRE tunnels on the router, with equal cost routes pointing down each tunnel. On the "other side" of the tunnel, you can place route-maps which direct the traffic out of the correct interface. I've just tested this on a 2811 with 12.4(2)T1 ADVEK9 and it appears to work.

Very interesting scenario you have. Please let us know if you get it working, as I'm quite curious to see if this can be done.

New Member

Re: Load Balancing across 4 low cost DSL links

Actually, giving this some more thought, the above isn't required - I think it can be done on a single router.

The trick would be to use "ip nat inside source" with route-maps which make use of "match interface" statements, so that the NAT will select the correct interface IP address. I'm reasonably sure this will work even with conditional routes/tracking.

New Member

Re: Load Balancing across 4 low cost DSL links

Hi ,

I have tested a scenario using a layer 3 device capable of PBR behind the outgoing internet routers, the internet routers with the DSL links have equal path statics with tracking and because the default load-balancing is destination based it works just great.

But this is in lab phase for now so lets see what else unfolds

Thanks for your input