Agent is Displaying Offline in the Core UI but the Agent Service is Running
During random circumstances the Agent will appear offline in the Core. Before it goes offline some of the backups are more than likely failing. To bring back the connection to the Agent a Core server reboot needs to be done.
The Core cannot communicate with the Agent because the Maximum Transmission Unit (MTU) value is too large for the Agent to handle.
First, on the Core ping the Agent in the following format: ping AgentIP -a -l 1500.
NOTE: The switches -a resolves the IP to hostname and -l is the size of the packet being sent.
If the Core machine can't send and receive requests larger then 1472 bytes you have to reduce the default MTU value (1500 bytes) on the Core machine to the custom 1400 bytes value in a cmd prompt with the following command:
To show the MTU value: netsh interface ipv4 show interfaces
To change the MTU value: netsh interface ipv4 set subinterface "Local Area Connection" mtu=1400 store=persistent
An MTU limit that is set too high can cause fragmented packets and packet loss on the connection. This allows the Core machine to divide large packets into smaller ones. It solves the connection problem between the Core and the Agent, but it does not resolve possible network issues in the environment. They could be caused by some hardware or software firewall, a switch or router configuration, or any physical problems with the network cable.