To avoid confusion with alternative route search we talk not about ALT but about
the 'landmarks' algorithm. The landmarks algorithm is like CH a post-import
process which makes routing faster. It is disable by default. To enable it
add the profiles for which you want this to profiles_lm in the config.yml.
The interesting part is that we set a special landmarks WeightApproximator for (bidirectional) A* and are done. It is important that although a weight approximation is used the landmarks algorithm is still not heuristic per default. You can make it faster and more heuristical behaving when you set the epsilon parameter lower than 1.
Currently we limit routing requests using the landmark algorithm, then it is not possible to cross the borders between EU, Africa, and Asia. All other algorithms are not affected by this limitation. For landmarks this limitation is important as we store the weight approximation in a short (two bytes) and for large distances more bytes would be necessary but e.g. for the world wide case this would mean several additional GB per weighting.
Furthermore this short value has the problem that smaller distances cannot be differentiated sometimes which could lead to very slightly suboptimal routes. If just small areas (country or city) are imported this can be avoided via setting a the maximum weighting explicitly if no good default was found.