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It’s rare these days to encounter computers or applications that don’t in some way rely on a network connection. General networking principals apply to virtual machines just as they apply to physical ones. But, since the host’s network adapters will be providing resources for all VMs, there are some special considerations, as well. In this article, I’ll present some ways in which you can design networks with virtualization performance in mind.

Note: For an introduction to working with Virtual Server’s networking options, see Configuring Virtual Networks in Virtual Server – Microsoft Virtual Server from the Ground Up.

Managing Host Network Adapters

When multiple VMs perform network-intensive operations, the host’s network adapter can become a bottleneck. In the simplest network configuration, a Virtual Server host computer will have only physical network port. While this system will allow you to share the network adapter with VMs, you can add some security and manageability by adding a second network port. Figure 1 shows an example. Since it’s a good idea to isolate network traffic (for security and performance reasons), you can choose to place all VMs on a separate Virtual LAN (VLAN) on your switch.


Figure 1: Using multiple host network adapters.

Since Virtual Server allows you to connect up to four virtual NICs per VM, you can add additional physical network connections, as needed.

Using the Virtual DHCP Server

Virtual Server’s built-in DHCP Server can be enabled for virtual networks and can help you create logically-separate networks on the same physical network segments. Through the use of differing IP address ranges, this technique can help segregate network traffic without requiring the configuration of VLANs or other devices on your switches. Figure 2 shows an example of a potential configuration.


Figure 2: Using DHCP to logically isolate network traffic.

Using NIC Teaming

The concept of NIC teaming is to allow multiple network ports to act as one logical unit. There are two main goals. The first is automatic fail-over. If one of the connections becomes unavailable (due to a port or switch failure), the other port can seamlessly take over the load. The other goal is performance: Having multiple ports working together in a group can increase effective bandwidth. Keep in mind that some configurations will depend on support from the network infrastructure-side (for example, port grouping options on switches).

Other network adapter optimizations include changing the default TCP packet size. If you’ll be routinely transferring large files (such as VHDs) between servers, using Jumbo Frames can greatly reduce overhead and increase performance.

Virtual Server and Firewalls

It usually goes without saying that firewalls and port-level filters provide an important layer of defense for Virtual Server hosts and VMs. Theoretically, if an unauthorized user gained access to your Virtual Server host, she could gain access to the VMs themselves. If you want to place a firewall between Virtual Server and potential users, you’ll need to keep in mind which ports you might need to open to make various services available (see Figure 3).


Figure 3: TCP Ports used by various Virtual Server-related services

Monitoring Network-Related Performance

When planning for virtualization network configurations, it can be useful to get statistics about traffic traversing the host and guest interfaces. Figure 4 provides an example of statistics that can be collecting using Windows System Monitor. When measured at the host level, you can get an aggregate summary of how much bandwidth is being used and if there’s an outbound queue. To drill-down on the source of the network information, each guest OS can be monitored. You can further filter the details per network adapter.


Figure 4: Network performance counters of the “Network Interface” object


There are many different ways in which you can configure networks to better support virtual machines. We looked at methods for segmenting traffic, increasing throughput, configuring firewalls, and monitoring network statistics. All of this can help optimize network performance in virtualized environments of any size.