Hello,
For this part of the problem, I'm having troubles understanding why R2 does not receive announcements from B1 and B2 for the routes they learnt from E-BGP? I understand B1 and B2 won't select those routes as their best route but depending on the timing, couldn't we have that B3 and B4 give their updates to B1 and B2 a bit later and therefore, B3 and B4 would already have announced to R2 their Best Route to C (that in that case goes through A)? I'm not sure I understand the mechanism that would prevent this kind of behaviour to happen.
More generally, is there any specific propagation order we should assume for the I-BGP routes announcements?
Thanks for your help.