BGP repropagation practices on not well known attributes

Hello bird users, I'm sorry to bother you but I have a question about the implementation of a piece of the RFC 4271 that I found a little bit fuzzy. I'm in a eBGP situation and suppose that I have more connections with other different AS peers. Giving the fact that my import and export filters are completely open for simplicity and that I accept optional attributes. When I receive an UPDATE where it changes only a optional transitive argument that i do not know I re-propagate the UPDATE to my other connections as like a well known mandatory argument is changed? So my general question is, how Bird handle optional transitive attributes that a peer does not know? Thanks in advance, and sorry again to bother you, Mattia