It will work with the 30 just fine.

6700 has a maximum system difference of 33 teeth.
That is the difference between the big/big combo and the little/little combo.

The 6700 is not rated for a 30T cassette.
It is rated for a 28T

If you run the 12-30T, shifting will not be great as you have swing the derailleur far away from the cluster for it to clear the 30T cog.
It can be run however, but you need to be aware of the compromises to go outside of Shimano's official specs.

If you are running a compact crank then that is 16T difference right there.
12-32 gives you another 20 for 36T in total which is far outside of the 33T spec.
So the chain will go slack in the small small combo's and the chain may even rub on itself as the derailleur cage swings so far back.

If you don't use the cross chain gears then you should be fine, but remember there may be consequences to doing this and you need to be mindful of which gear you are in.

Thanks for all of the detailed info Lyrrad! Maybe it would be best for me to get a 28/11 and keep it within what is recommend. That should still help quite a bit on steep climbs compared to the 25 I would think, but as you can tell I have no experience with this.

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"If it costs you 30 minutes at Maryland so what" -dwreal

Using a 28T cog for climbing will make a noticable difference compared to a 25T

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res, non verba

Cassettes are usually listed with the smallest cog first (i.e. 11-28).

It all depends on the bike. Some have longer derailleur hanger lengths than others. Usually you can stretch the recommendations though. Shimano's are conservative. I would suspect you'd have no problems, but you never know until you try.

I have run an 11-36 on my Cannondale with a standard mid-cage 6700 for example (rated for 30t—yes some of them are). It doesn't shift amazingly with that combo, so it's not my first choice, but if I need for a specific race I wouldn't hesitate to use it.

At least when you experiment with different cassettes, it's a cheap and simple swap to go back. And if you're doing a new chain too, it's much easier to remove links if your experiment isn't sucessful.

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George Dedopoulos | @geodee | geodee.com | Team Atomica | Toronto Triathlon Club

He is running 10 speed so there is only long cage and short cage.
If his bike come with a 25t you can guarantee it is not a rare one that come with a long cage.
To run the cluster he wants exceeds the total capacity.
The chain will either be too short and risk breaking the derailleur or will not have tension in the small small combo or even the next couple of gears.
There is no stretching the total capacity, it is either yes or no, there is no grey maybe unless you simply do not shift into those gears.
His problem is made worse by having a compact crank which has larger tooth difference than the more standard crank that most TT bikes are using.
You can run the derailleur on a 30t rear cog with less than optimal shifting, but the cage is simply too short to take up 33 T of difference.
I wouldn't condone anybody setting up a potential derailleur break or chain drop for anybody in the OP's position of not totally understanding the caveats.
Especially when the bike in question is a short chainstay bike that will drop chains easily with no tension and struggle to run properly with the hell stretched out of the derailleur.

It's interesting that the Ultegra 6700 has a max of 28 teeth but the 105 5701 has a max of 30 and a 34 tooth capacity which can accommodate a 50/34, 12-30

http://bike.shimano.com/...51/rd-5701-ss-l.html

(I know the link says 11 but it's really the 10 speed SS page)

There is a 30t version of the 6700, the 6700-A. The mid-cage (long-cage) version (GS) can handle a 40t differential, so it can run a compact with 11-30 too.

The only question is if the OP is using a short-cage or mid-cage, as as the other poster mentioned, it's probably a short-cage version.

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_____________________________________________________
George Dedopoulos | @geodee | geodee.com | Team Atomica | Toronto Triathlon Club