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New Member

Multiple Internet providers

I currently have 2 T1's going to the Internet. Each T1 is with a different Internet provider. Is it possible to use both T1's (load balancing) for Internet access? And if one of the Internet providers goes down it needs to kick over to the other provider. I'm running a 3640 as an external router to both circuits.


Brian Sowle

  • Other Network Infrastructure Subjects
Cisco Employee

Re: Multiple Internet providers


It all depends on exactly what you mean by "use both T1's (load balancing) for Internet access". ;-)

In general, there's no easy way to load share traffic, specially incoming...because you really don't have a lot of control over what other autonomous systems do. The straight forward mechanism would be to use AS_PATH prepend to influence the return traffic.

For outgoing traffic it may be easier because you have control over what happens in your network. In your case (two links out of the same router), the easy solution for outbound load sharing is to just put two defaults, one pointing at each provider. This, of course, assumes that both providers have pretty much the same reachability. If you want to be really safe (may be overkill in some situations), then accept full tables from both providers and configure BGP to install at least 2 paths in the routing table (maximum-paths). The caveat with this last solution is that it requires memory to store all that routing information.

Until a better solution is available, you're pretty much stuch with manual configuration (as above). In future IOS releases we should be able to help a little bit more. Take a look at the OER slides in my Networkers presentation from this year:



New Member

Re: Multiple Internet providers

I have a similar situation with 2 T1s fro 2 different providers going into 2 separate routers. I am running one internal mail server to which I assigned 2 IP addresses, one from each segment associated with each T1 (64.x.x.m, and 68.y.y.n). I am running W2K Advanced server with DHCP with 2 scopes (1 scope per segment) and a multiscope. Once the first scope of IP addresses (68.x.x.X) has been assigned, the multiscope then assigns addresses from the second scope (64.y.y.Y). The scope ranges have equal number of available IPs, and that allows me to segregate my network between the 2 T1s and more or less have the same number of customers on each T1.

However, I am thinking of adding a 2nd Ethernet card to the 64.y.y.1 router and assign to it an IP address from the other segment 68.x.x.2 and add an ip route to the network. I will then have 2 ip routes in the router, 64.0 and 68.0. Will this load balance outgoing traffic between the 2 T1s?

Thanks for your input.


New Member

Re: Multiple Internet providers


I hope there is one more solution for this same problem which we were facing since very long time. You can youse RADWARE Linkproof or load balancer equipment for load balancing among multiple ISPs. It can work well by maintaing the load, traffic, number of user sessions for each WAN link to your ISP as well it continuously pools the far end of the link and keeps its NHR (Next Hop Router) table that the link is alive or dead. Presently we are using 2 ISPs of different speeds. Even you can also prioritise which link is preffered based on the type of your out going traffic like smtp or www etc. You can get more details on radware website for Linkproof product. Its quite stable and proven i believe. But any way I fully agree that incoming load balancing is not possible as you have no control over the ISP routing. But again there is one indirect way of achieving i can have your mail servers, web servers published on both ISP's and have your DNS server resolving to both IP address ranges provided by your ISPs for the same severs.

You can publish your web servers or mail servers or any other severs which needed to be accessed from out side with two different Public IP addresses on your Linkproof and then same IP you can get it registered for DNS resolutions. It works pretty well though we can't get assured load balancing for incoming traffic but you can have your servers available even though any one link fails.

If anybody disagree please let me know and correct my statement. This i was found upto my little experience and observation

Best Regards,

Zakir Hussain

CMC Limited


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