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2 T1 - 1 2621


We are about to have a 2nd T1 into the building. I have asked questions previous to this and now need some advice on how to proceed. I would like to know if it is possible to untilize both T1s equally when up and if one fails to continue using the other. I plan to be conecting the T1s via serial. The FE port is connected to my server. What would be the best way to achieve what I am attempting?

Thanks in advance.



Re: 2 T1 - 1 2621

Hi Paul,

The simplest option is to configure two equal-cost static routes and to enable CEF.

The config required is like the following:

ip cef


ip route serial0:1

ip route serial1:1

The use of CEF will allow you to load-share over the two links. The use of two equal-cost static routes will ensure that both links will be available for use but if one goes down, the other will continue to be used, giving you redundancy.

Hope that helps - pls rate the post if it does.


Re: 2 T1 - 1 2621

Thanks for your reply, it was very helpful. Would the configuration you are suggesting utilize all available bandwidth? I seem to remember reading that the link would only be as fast as one link. I may be getting confused!


Re: 2 T1 - 1 2621

Load-sharing of any kind depends on the nature of the traffic. In case of CEF, it does load-sharing on a per-destination basis by default. You can configure it to perform per-packet load-sharing which gives you better load balancing but may possibly result in out-of-order packets which is why I would not recommend it.

With the default setup, however, there is no reason why you will not be able to utilise the full bandwidth of the 2 T1s, limited only by the CPU hit taken by any other features you may have enabled.

There is one thing, however: when using CEF, a single flow will never be split over two different links. Therefore, the max rate you will get for a single flow will be limited to one T1, in your case. However, in most cases, you will have a mix of traffic so both links will get used to their full extent.

Hope that helps - pls rate the post if it does.