Shaping a link

Unanswered Question
Mar 18th, 2009

All,

I have a 20mb DS3 that's getting installed at a remote branch. We have a 20mb DS3 circuit here as well, and we don't want the remote branch being able to suck down all of our bandwidth.

I can shape the traffic on the outside interface for the remote branch, but if I shaped it from 20mb to 3mb, would that take a major hit on the routers cpu? It's a DR site, and the only time we want them to be able to use 20mb is if we had to go over there because of a DR situation. The router is a 3745, and there are about 100 users at this location.

Thanks,

John

I have this problem too.
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Jon Marshall Wed, 03/18/2009 - 08:56

John

Shaping does incur extra CPU overhead but it is really quite minimal and if adding shaping to the router has a major impact on CPU then you probably need a better router anyway.

With what you have you should be fine.

Jon

Paolo Bevilacqua Wed, 03/18/2009 - 09:55

Which exact type of circuit is that? If p-t-p DS3 you can use dsu commands to limit bandwidth.

Joseph W. Doherty Wed, 03/18/2009 - 10:11

As the others have noted, yes you could and CPU is (usually) minimal. (re: usually, I've noticed on the later 12.4T versions, the default Tc now appears to be 4ms and I've seen such a minimal value impact the CPU. [If it does, increasing Tc appears to reduce CPU impact.])

PS:

As an alternative, instead of always limiting the bandwidth to the remote site, you could also consider using something like CBWFQ with a 15% bandwidth setting. This should also provide 3 Mbps but will allow more if not otherwise being used.

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