BGP repropagation practices on not well known attributes

[Mattia Milani]

> First, handling of optional attributes depends on whether you knows them,
> not on whether a peer knows them.
>
> Unknown optional transitive attributes are re-propagated, but with set
> 'partial' flag. For details, see bgp_export_attr() function.

Thanks for the answer I really appreciate it.

Yeah form the RFC I get that unknown optional transitive attributes are 
re-propagated.
I was more interest in acceptance part, the RFC 4271, section 5, 5th 
paragraph says:

"... Paths with unrecognized transitive optional attributes SHOULD be 
accepted. ..."
This SHOULD was confusing me.

This take me to an other question,
Given that I already know the destination d (that i learnt with an 
unknown optional transit attribute that I re-propagated with the 
'partial' flag).
At some point I receive an UPDATE for the destination d, and the only 
attribute that is different is the unknown optional transit one.

At this point a bird node, in first place can recognize that only the 
unknown optional transit attribute has been changed? and in addiction, 
if it recognize it, the UPDATE will be re-propagated with the optional 
transit attribute and the flag 'partial', am I right?

Thanks for the help,
Mattia