Add MTU limit requirements for custom NVAs

Author: thisisanniefang

articles/load-balancer/tutorial-create-gateway-load-balancer.md

@@ -548,6 +548,8 @@ New-AzLoadBalancer @lb
 ---
 
 ## Add network virtual appliances to the Gateway Load Balancer backend pool
+> [!NOTE]
+> If leveraging your own custom network virtual appliance in the backend pool of a Gateway Load Balancer, please ensure the MTU of all NVA virtual machines are raised to a minimum of 1550 bytes to accomodate for the VXLAN encapsulated headers. This will allow source packets up to the limit of 1500 byte packets in Azure, avoiding fragmentation.
 
 # [Azure portal](#tab/azureportal)
 
@@ -831,4 +833,4 @@ When creating the NVAs, choose the resources created in this tutorial:
 
 Advance to the next article to learn how to create a cross-region Azure Load Balancer.
 > [!div class="nextstepaction"]
-> [Global load balancer](tutorial-cross-region-portal.md)
+> [Global load balancer](tutorial-cross-region-portal.md)