Shimano Ultegra CS-6700 Bicycle Chain and Cassette - 10 Speed Grey 11-25T | ProBikeKit UK
How to tell cassette is worn? been running this 11-28 ultegra casette for at least 7000km. No shifting issues or weird noises. I do replace chain every 3500/4000km depending on chain wear
Upgrading Your Bike from a 10- to 11-Speed Cassette - Road Bike Rider Cycling Site