A tool you can use to evaluate your network is a modified ping test. Using a ping with the options -f -l and setting the packet size can be used to test the end devices and network to determine the largest frame size supported. In Figure 4, you can see three ping tests.
Note: When setting the packet size in a ping test, you set the packet size at 12 bytes greater than the desired MSS.
Figure 4: Ping tests showing different MSS results
In the first two pings, sent over my WAN connection to smallnetbuilder.com, you can see in Figure 4 that I received the fragmentation warning with a packet size of 1472 bytes, but succeeded at 1464 bytes. This highlights one more common frame limitation; various circuits have different MTU sizes. My ISP connection utilizes PPPoE, which has a maximum MTU of 1492 bytes due to the need for an additional 8 bytes of header space. In the third ping, sent over my LAN, I was able to ping my gateway's interface using a packet size of 1472 bytes. This same methodology can be used to run tests with larger packet sizes over gigabit Ethernet LANs.
Once you've identified the highest packet size that works with a ping, do some testing between devices. Figure 1 showed a PC with an Intel NIC that supports frame sizes of 4088, 9014, and 16128 bytes. Your NAS options may list 4k and 9k frame sizes. In this scenario, try sending a single file to and from PC and NAS multiple times using the 4088 byte setting on the PC and the 4k setting on the NAS. Repeat the test using the 9014 and 9k settings to determine the average throughput. Throughput can be calculated by taking the file size and dividing by the seconds to transfer the file. You can do the calculation in MBytes/sec or Mbps (M bits per second); just be consistent.
As you may find, the largest size supported by both devices may not necessarily result in the fastest throughput. Limitations in PC hardware and software, as well as network hardware, can result in higher drop rates at higher frame sizes, causing excessive retransmissions and leading to lower throughput. Most of our testing on SmallNetBuilder is done using 4k jumbo frames because that's what works best with our computers and their 32 bit PCI NICs.
Some people recommend using only networking components from a single manufacturer when implementing jumbo frames. While this might seem like an easy way to go, it's questionable that there is any value in it. As long as all components support the same desired maximum jumbo frame size, they should work just fine.
A more important consideration is how to configure networks that have a mix of gigabit, gigabit + jumbo frame and 10/100 Ethernet devices. This is common in smaller networks where "forklift" gigabit upgrades of all devices are unlikely. You also have some devices such as phones and networked media players that aren't available in gigabit flavors.
Two approaches to mixed networks are to group devices physically or logically. If there are specific devices and traffic flows that merit jumbo frame transmission, isolating them to their own network is a means of leveraging jumbo frame throughput. This can be done on a physical device basis, or logically by separating jumbo frame devices into their own VLAN.
In the physical model depicted in Figure 5, the jumbo frame devices on the left are on a Layer 2 switch that supports gigabit Ethernet and jumbo frames, while the non-jumbo frame devices on the right are on a different Layer 2 switch. The two switches are connected to a router. Even if the link to the router from the jumbo frame LAN is only Fast (100 Mbps) Ethernet, the traffic between devices on the jumbo frame LAN can utilize jumbo frames safely.