Enhance FastPath section with requirements and features

Added details on FastPath requirements and advanced features.

Author: staleycyn

learn-pr/wwl-azure/design-implement-azure-expressroute/includes/9-improve-data-path-performance-between-networks-expressroute-fastpath.md

@@ -1,7 +1,10 @@
-
-
 ExpressRoute virtual network gateway facilitates the exchange of network routes and directs network traffic. [ExpressRoute FastPath](/azure/expressroute/about-fastpath) enhances data path performance between your on-premises network and your virtual networks. When enabled, ExpressRoute FastPath routes network traffic directly to virtual machines, bypassing the ExpressRoute virtual network gateway. 
 
+To use FastPath, your ExpressRoute virtual network gateway must be one of the following SKUs. A gateway is still required to exchange route information. FastPath bypasses the gateway only for data traffic.
+- Ultra Performance
+- ErGw3AZ
+- ErGwScale (minimum 10 scale units)
+
 FastPath is available on all ExpressRoute circuits. ExpressRoute FastPath is useful for enterprises that need consistent and high-performance connectivity to Azure for mission-critical applications. 
 
 :::image type="content" source="../media/fastpath-vnet-peering.png" alt-text="Diagram of the FastPath layout.":::
@@ -18,4 +21,11 @@ FastPath is available on all ExpressRoute circuits. ExpressRoute FastPath is use
 
 - **Reliability**. With a more direct connection, there's less chance of network congestion or packet loss, improving the reliability of data transfers.
 
-- **Security**. FastPath ensures that data travels through fewer intermediary points, potentially reducing exposure to security risks.
+- **Security**. FastPath ensures that data travels through fewer intermediary points, potentially reducing exposure to security risks.
+
+### Advanced FastPath features (ExpressRoute Direct only)
+
+When using FastPath with an ExpressRoute Direct circuit, three additional capabilities are available:
+-	**Virtual network peering**: FastPath sends traffic directly to VMs in spoke virtual networks connected via VNet peering. Hub and spoke virtual networks must be in the same region.
+-	**User-Defined Routes (UDRs)**: FastPath honors UDRs configured on the gateway subnet while maintaining the performance bypass.
+-	**Private Link and private endpoints**: FastPath sends traffic directly to private endpoints in spoke virtual networks. This feature is in limited general availability and requires enrollment.