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VMware ESXi5 SplitRX July 17, 2012

Posted by vbry21 in VMware blogs.
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I got asked a question last week regarding the ESXi SplitRX functionality, the question went along these lines “What’s SplitRX, I’ve seen it in a couple of sample vSphere exams?”

So here’s the answer.

Multicast is an efficient way of disseminating information and communicating over the network. A single sender can connect to multiple receivers and exchange information while conserving network bandwidth. Financial stock exchanges, multimedia content delivery networks, and commercial enterprises often use multicast as a communication mechanism. Multiple receivers can be enabled on a single ESXi host. Because the receivers are on the same host, the physical network does not have to transfer multiple copies of the same packet. Packet replication is carried out in the hypervisor instead.

SplitRx mode is an ESXi feature that uses multiple physical CPUs to process network packets received in a single network queue. This feature provides a scalable and efficient platform for multicast receivers. SplitRx mode typically improves throughput and CPU efficiency for multicast traffic workloads.

SplitRx mode is supported only on vmxnet3 network adapters. This feature is disabled by default. VMware recommends enabling splitRx Mode in situations where multiple virtual machines share a single physical NIC and receive a lot of multicast or broadcast packets.

SplitRx mode is individually configured for each virtual NIC

To enable SplitRX do the following

This feature, which is supported only for VMXNET3 virtual network adapters, is individually configured for each virtual NIC using the ethernetX.emuRxMode variable in each virtual machine’s .vmx file (where X is replaced with the network adapter’s ID).

The possible values for this variable are:

 ethernetX.emuRxMode = “0”

The above value disables splitRx mode for ethernetX.

ethernetX.emuRxMode = “1”

The above value enables splitRx mode for ethernetX.

To change this variable through the vSphere Client:

1 Select the virtual machine you wish to change, then click Edit virtual machine settings.

2 Under the Options tab, select General, then click Configuration Parameters.

3 Look for ethernetX.emuRxMode (where X is the number of the desired NIC). If the variable isn’t present, click Add Row and enter it as a new variable.

4 Click on the value to be changed and configure it as you wish.

The change will not take effect until the virtual machine has been restarted.

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