BGP repropagation practices on not well known attributes

[Mattia Milani]

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