The error of network uplink redundancy lost is a very common issue with VMware ESXi hosts. It occurs when the secondary network interface is not connected or in a degraded state to the ESXi host. An alarm reports a loss of uplink redundancy on a vSphere standard or a distributed switch for a host.
At this point, the Host is connected with the primary NIC and there is no impact but if the primary adapter fails then there would be an outage as there is no network adapter to carry out the network traffic. Your host becomes isolated.
For example, assume that a host in your environment has physical NICs vmnic0 and vmnic1 connected to vSwitch0 on host esx01, and the physical NIC vmnic1 goes offline or down, leaving only vmnic0 connected to vSwitch0. As a result, the uplink redundancy for vSwitch0 is lost on the host esx01.
Possible Solution For Network Uplink Redundancy Lost
First, you need to check that which vSwitch has lost the uplink redundancy on the host. You need to connect at least one more physical NIC on the host to this vSwitch and acknowledge the alarm to green. This issue might be lost for a few seconds and could be intermittent network loss on the physical switch. In this case, you need to check and verify with your network support team.
If a physical NIC is down, Request to your Data Center Operation team to check physically to the physical NIC and connect the NIC properly.
You also can analyze the events or tasks for the Host which is complaining about this, and look into the Tasks & Events tab.
To check the connected NICs details using esxcli use below commands:
esxcli network nic list
The above command will provide you a list of network adapter with their status as up or down and duplex speed.
If you want to check the network errors on any particular physical network adapter(vmnic1 in this case), use below commands:
ethtool -S vmnic1 | grep -i error
The ethtool is the best tool to check the physical network adapter information.
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