Update expressroute-routing.md

Added documentation that IANA reserved ASN numbers for documentation 64496 - 64511 are not allowed in the path for Microsoft Peering

Author: davegoddard

articles/expressroute/expressroute-routing.md

@@ -94,7 +94,7 @@ Make sure that your IP address and AS number are registered to you in one of the
 
 If your prefixes and AS number are not assigned to you in the preceding registries, you need to open a support case for manual validation of your prefixes and ASN. Support requires documentation, such as a Letter of Authorization, that proves you are allowed to use the resources.
 
-A Private AS Number is allowed with Microsoft Peering, but will also require manual validation. In addition, we remove private AS numbers in the AS PATH for the received prefixes. As a result, you can't append private AS numbers in the AS PATH to [influence routing for Microsoft Peering](expressroute-optimize-routing.md). 
+A Private AS Number is allowed with Microsoft Peering, but will also require manual validation. In addition, we remove private AS numbers in the AS PATH for the received prefixes. As a result, you can't append private AS numbers in the AS PATH to [influence routing for Microsoft Peering](expressroute-optimize-routing.md). Additionally, AS numbers 64496 - 64511 reserved by IANA for documentation purposes are not allowed in the path.
 
 > [!IMPORTANT]
 > Do not advertise the same public IP route to the public Internet and over ExpressRoute. To reduce the risk of incorrect configuration causing asymmetric routing, we strongly recommend that the [NAT IP addresses](expressroute-nat.md) advertised to Microsoft over ExpressRoute be from a range that is not advertised to the internet at all. If this is not possible to achieve, it is essential to ensure you advertise a more specific range over ExpressRoute than the one on the Internet connection. Besides the public route for NAT, you can also advertise over ExpressRoute the Public IP addresses used by the servers in your on-premises network that communicate with Microsoft 365 endpoints within Microsoft.