OSDOCS-13187:Manually setting the MTU value

Author: Ted Avery

modules/microshift-nw-topology.adoc

@@ -9,6 +9,11 @@ OVN-Kubernetes provides an overlay-based networking implementation. This overlay
 
 To configure the MTU, you must set an equal-to or less-than value than the MTU of the physical interface on the host. A less-than value for the MTU makes room for the required information that is added to the tunnel header before it is transmitted.
 
+[IMPORTANT]
+====
+The MTU value of the OVN overlay networking in {microshift-short} must be 100 bytes smaller than the MTU value of the base network. If no MTU value is configured, {microshift-short} autoconfigures the value using the MTU value of the default gateway (Internet Protocol version 4 (IPv4) or Internet Protocol version 6 (IPv6)) of the host. If the auto-configuration does not work correctly, the MTU value can be configured manually. For example, if the MTU value of the network is `9000`, the OVN MTU size must be set to `8900`.
+====
+
 OVS runs as a systemd service on the {microshift-short} node. The OVS RPM package is installed as a dependency to the `microshift-networking` RPM package. OVS is started immediately when the `microshift-networking` RPM is installed.
 
 .{product-title} network topology