just a quick question. now i know halo 3:odst has the full multiplayer part from halo 3 including the dlc, just wanted to know if halo cea has the same thing where the multiplayer has the full reach multiplayer.
To my knowledge, you are just getting the Anniversary Map Pack for Reach when you buy Anniversary.
This map pack features six remade multiplayer maps from Halo: Combat Evolved and Halo 2. Some of the layouts, weapon drops and vehicles have changed to fit the Reach game play, but you can get the original maps for forging.
Multiplayer does use the Reach Engine though.
=d
Only Anniversary maps.
You can only play on the Anniversary maps.
You can Forge, and play Custom Games on the Anniversary maps. As well as play 2 Reach online matchmaking playlists; Anniversary Classic and Anniversary Firefight.
Hope I helped 
Anniversary Map Pack, six matchmaking maps and one firefight map. Otherwise, everyone has stated what’s available and I shall go.