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Jumbo frames are a valuable tool for virtualized environments. Traditional Ethernet frames, packets, contain up to 1,500 bytes, but each packet includes additional content for a header, checksum and end. And each frame must be acknowledged -- adding traffic and latency which slows effective network communication. Boosting traditional Ethernet frames from 1,500 to as much as 9,000 bytes packs far more data payload into each frame and eases the latency caused by frequent handshakes. These larger frames are collectively known as jumbo frames. Hypervisors like VMware ESXi enable jumbo frame support, but it's important to verify that each part of the network is properly configured to support it.
The key to jumbo frame support is end-to-end support in the physical network hardware. This means the server's network adapter -- either a network interface card (NIC) or LAN-on-motherboard adapter -- must support the desired jumbo frame size, along with any network switch ports and Ethernet storage subsystem network ports. Each element of the network must be configured to use the same jumbo frame size, otherwise the misconfigured component will not communicate with the rest of the network. Once the hardware devices are properly configured, administrators can configure jumbo frames for each ESXi vSwitch using VMware's command-line interface on each virtualized host system.
Remember that some switches may default to use payload or maximum transmission unit (MTU) settings higher than the typical jumbo frame size. For example, a Cisco Nexus 5000 Series switch defaults to a jumbo MTU of 9,216 bytes, so this may need to be changed manually in order to function correctly with other network devices set for jumbo frame operation because ESXi only supports a maximum MTU of 9,000 bytes.
Use caution when employing jumbo frames with network adapters designed for iSCSI offload functions because the two features may not be compatible on some NICs. For example, VMware identifies possible problems using jumbo frames with Broadcom cards serving as iSCSI initiators with offload capabilities. The adapter can service jumbo frames or iSCSI offload -- but not both together. The solution in such cases may be to upgrade the problematic NIC to a later model which overcomes this issue.
Actually enabling jumbo frames for network adapters can be accomplished through the vSphere Web Client by locating the host system and then opting to manage networking or storage -- depending on whether you're configuring vSwitches or hardware devices respectively -- selecting the switch or adapter, opening its properties or advanced options details, and then configuring the MTU value. Remember to save any changes and document any configuration changes which can be undone later if necessary.
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