Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
I am afraid I did not understand your question, can you please elaborate.
Route advertisement is automatic in an ExpressRoute scenario.
- See : How are virtual networks advertised on ExpressRoute private peering?
- This means, every subnet will be advertised to the peer via the ExR.
Are you saying that the BGP routes advertised or from a different range than that of the VNETs' ?
Cheers,
Kapil