Update route-server-faq.md

I am a support engineer from CSS, and we have received many questions regarding this. Customers advertise AS path prepend  from NVA to route server and expect to see it on their on-premises routers. Could you review and update the public documentation?

Author: shyambasnet1

articles/route-server/route-server-faq.md

@@ -135,6 +135,13 @@ No, Azure Route Server provides transit only between ExpressRoute and Site-to-Si
 
 No. The spoke VNet can't have a Route Server if it's connected to the virtual WAN hub.
 
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+### If Network Virtual Appliance has AS path prepend configured towards the route server, will the ExpressRoute circuit pass the AS path prepend information to the on-premises router?
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+No. When ExpressRoute advertises routes to on-premises, it removes the private BGP ASN, and you will only see the ExpressRoute circuit ASN
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 ## Limitations
 
 ### How many Azure Route Servers can I create in a virtual network?