2811 - 8xT1 IMA?

Unanswered Question


We currently have a 2811 router with two 2 port T1 WICs. These are configured in a single IMA group as a 6Mb internet circuit.

We would like to double our capacity by adding two more 2 port T1 WICs. From my reading, we can't create an IMA group that has more than 4 T1s. The Cisco documentation says that "two ATM AIMS can be used for 8xT1 IMA, but they must be configured as two 4xT1 IMA groups."

How does this work, and is there a configuration that would allow us to use 12Mbs? Our internet provider doesn't think they can make it work on this router. Are we looking at having to upgrade the router, or is there a configuration that would let us bond the two IMA groups? Would we have to have our provider give us two bundles of 4 T1s, each bundle having a separate external IP address?

If it's possible to make this work, do you have a sample of how that might look?

Sorry for all the questions, and many thanks in advance for any advice/help.


"Two ATM AIMs can be used for 8X T1 IMA, but they must be configured as two 4X T1 IMA groups."



I have this problem too.
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Paolo Bevilacqua Wed, 06/17/2009 - 10:17

If you use the T1 for regular internet, the best result are without any ATM and any IMA, just use CEF load balancing. This way you make the best use of the circuit bandwidth.

Joseph W. Doherty Wed, 06/17/2009 - 12:00

From documenation, it appears the AIM-ATM can only drive up to 4 ports. So, although two AIM-ATMs support 8 ports, it appears this is what forces 8 ports to be placed into two separate groups.

I suppose you could then use the two groups as two logical links.


If you can obtain it, you might find something like a 10 Mbps Ethernet hand-off is a better choice. Considering the overhead involved with ATM, 10 Mbps Ethernnet should deliver the about same throughput as 12 Mbps of ATM.

You might also price a fractional T3.

Paolo Bevilacqua Wed, 06/17/2009 - 12:41

Basically, anything but 8 T1s w/ ATM.

So many circuits, so many cables, so many interfaces and complications for so little bandwidth.

bofawcett Wed, 06/17/2009 - 15:12

Usually by the time you get to 4 T1s its just as cheap to go to T3.

Giuseppe Larosa Thu, 06/18/2009 - 01:02

Hello Bob,

yes you're right but for supporting a full rate T3 another router more powerful may be needed then 2811.

Probably the usage of ATM IMA could allow to overcome a limitation to 4 T1 typically associated to this router

(just trying to guess why someone would want to use ATM IMA) by managing two groups of 4 T1 ports with additional AIM modules.

However, it can lead to problems.

Hope to help


Joseph W. Doherty Thu, 06/18/2009 - 04:58


I've worked with a 2811 that has a full T3. It has been able to push traffic up to about 20 Mbps, duplex, but that's running at 100% CPU. Most of the time on that router, WAN traffic doesn't demand more than 10 to 15 duplex, and although still high CPU, it doesn't max out and handles the load. If you wanted to fully drive a T3, the 3825 seems to do well (from T3 experience - NB: has about 3x pps of 2811), although a 2851 might come very close (no experience with T3 - NB: has abut 2x pps of 2811).


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