image from manufacturer
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The iRobot Roomba 871 is sold at Japan only at this moment, you can find it here http://store.irobot-jp.com/item/871.html. It is very similar to the Roomba 870, same AeroForce Performance Cleaning System, HEPA filter and Bin, Xlife Battery, Integrated Home Base, NO Lighthouse, etc. The only difference between Roomba 870 and 871 is the accessories they come with.
The 870 comes with 2 virtual walls and 1 extra HEPA filter whereas the 871 comes with 1 virtual wall and no extra filter.
Otherwise, they are the same.