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New Member

Cisco 1721: Load balancing across multiple internet T1's...

I have a customer with an existing Cisco 1721 equipped with a WIC-1DSU-T1 for internet access. The customer wants add bandwidth with an additional T1.

The ISP they have been talking to says it's as simple as adding the new T1, adding another WIC-1DSU-T1 to the 1721 and configuring it to load balance across the two T1's.

I don't see how that will work, as I assume that the two T1's would be in different IP blocks, etc. I thought that we would have to use external CSU/DSU's with Mux/Demux capabilities (multiple T1 ports) and connect this to a high-speed serial sync port on the 1721 and the ISP would have to do the same on their end.

Am I making this more complex than it needs to be? Is their some "magic" configuration for the 1721 that accomplishes what the ISP describes? If so, where can I obtain an example or white paper?

Finally, does the 1721 have the "horsepower" to accomodate multiple T1's or should we be using a more robust product?

Thanks in advance for the enlightenment!

Ken Morley

  • Other Network Infrastructure Subjects
New Member

Re: Cisco 1721: Load balancing across multiple internet T1's...

Talk to your ISP about CEF. I have setup several sites with load balancing Ts using CEF. It only takes a few commands. And works well. Do a search on CCO for CEF. I have been using 2600 routers and with multiple Ts with no performance impact not sure about the 1721.

New Member

Re: Cisco 1721: Load balancing across multiple internet T1's...

Yes, this is very easy to do. You are correct that each T1 will be in different IP blocks, but that doesn't matter because your trying to get to the Lan which is in its own IP block. Both T1s will advertise to the routing protocol that they can both get there. You just need to ensure that the ISP treats both paths with the same cost so it will balance.

If you wanted you can also do the mux/de-mux that you mentioned internally on the router using a Virtual dialer interface to handle the IP stack. You would not need to use any external DSUs.

You should not have any issues running 2 T1 on a 1721. However, you have now used up all your WIC slots. So if you ever wish to add something like an ISDN line for backup you will need a router with more ports. The 1721 also doesn't support technologies like VoIP and IP Telephony.

Happy Routing & Merry Christmas,



New Member

Re: Cisco 1721: Load balancing across multiple internet T1's...

I agree with all the solutions .See There can be two ways either you can go for routing protocols load balacing or you can use PPP multilink feature and do load balacing.(BY configuring rotary pool and Dialer interface).The another way round if you are using frame-relay then Cisco IOS itself in new ios images have included support for frame-relay load balacing.No need to rely on ppp encap multilink feature. or use can also go for CEF but i have never implemented that part.

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