The new Network Health Check feature in ESXi 5.1 tries to identify several different potential network configuration issues. In this blog post we shall study the MTU size test.
For information about the VLAN checks please see this post.
The Network Health Check is only available on the Distributed vSwitch version 5.1. Let us first use the new Web Client interface to enable large frame sizes, often called Jumbo Frames, for the virtual switch in general. A commonly used non-default frame size is 9000 bytes.
On this Distributed vSwitch version 5.1 there are two Virtual Machine port groups. They belong to VLAN 100 and 200. On both these port groups there could now exist VMs configured to use Jumbo Frames, but for the VM to successfully use Jumbo all physical switches end-to-end must support and allow large frame sizes.
We will now enable the new Network Health Check to see if there is any detectable issues with our MTU size configuration. These settings are found on the Manage tab of the Distributed vSwitch on ESXi 5.1.
The Network Health Check will now send jumbo sized broadcast frames through all interfaces for all VLANs and verify if they make it through. This will be repeated every minute. We shall note that the ESXi 5.1 host have no possibility to verify Jumbo Frames end-to-end. This must be done manually by VMKping with proper parameters. The Network Health Check can only try to verify the switch most close to the ESXi 5.1 host.
On the Monitor tab we get a quick warning that the ESXi host has detected some kind of issues with sending frames with the new large MTU size of 9000 bytes.
On the Health Status Details we could that the ports connected to vmnic2 and vmnic3 on the physical switch does not seem to allow Jumbo Frames. We see that both VLAN 100 and 200 are tagged on these interfaces.
In the physical switch (here HP Procurve 5406) we could verify the Jumbo Frames settings with the “show VLAN” command and we do indeed notice that neither VLAN 100 or 200 allows large frames at the moment.
This misconfiguration is clearly the cause of the Network Health Check MTU warning.
We will now configure the switch to allow Jumbo Frames on the VLANs. In the HP Procurve Command Line Interface the command is simple: “vlan 100 jumbo“.
This will now allow the ESXi test frames to pass through the VLAN 100 and 200.
Back in the ESXi 5.1 host and see the Network Health status. The Health Check does now confirm that the MTU configuration is correct on the physical switch.
The Jumbo Frames detection will help out a lot, but it can not detect end-to-end connectivity, i.e. from the ESXi host to the Storage Target, so still a manual confirmation with Vmkping will be needed.