Every vSwitch has two basic properties that can be configured in order to meet the requirements of your design and network’s maximum transmission size.
Ports indicate the number of virtual ports that will be kept in memory, tracked, and made available to VMs, VMkernel ports, and uplinks that reside on the host. One weakness of a standard vSwitch is the requirement that the ESXi host be restarted if you change the number of ports. Prior to vSphere 4.1, the default number of vSwitch ports was only 56, leading many a green VMware administrator to hit that limit before realizing it was something that could be changed. Over time, VMware listened to the woes of virtualization administrators and, in vSphere 4.1, the default number of ports assigned to a standard vSwitch has been changed to 128, allowing some breathing room. An administrator can adjust the number of ports by powers of 2, from 128 to 256 and so on, all the way up to 4,096 possible ports.
Figure 8.1 shows the default vSwitch properties dialog in the vSphere Web Client.
Figure 8.1 The default vSwitch properties
Maximum Transmission Unit (MTU)
The other item that you can configure is the MTU, which is the maximum amount of data that can be crammed into a frame’s payload segment. By default, this is 1,500 bytes, which is the default for just about any networking device you can buy. You can safely assume that all of the physical equipment that runs northbound of the vSwitch will support a 1,500 MTU or larger, which avoids unnecessary packet fragmentation.
There’s also an option to increase this size and set it to a “jumbo” size. We do love our silly names in this industry. Jumbo frames are just frames larger than the default size of 1,500. Even setting an MTU of 1,501 is technically enabling jumbo frames. Tremble before the mighty, slightly larger frame.
Most of the time, though, the term jumbo frame refers to a frame with an MTU of 9,000 or higher, though 9,000 is the maximum MTU ESXi will support. If you are talking to a network engineer and want to get an idea of what MTU size to set on your vSwitch, ask specifically what the MTU value is—don’t just ask if he or she is running jumbo frames. This avoids any confusion.