ifMainMtu 1.3.6.1.4.1.207.1.4.165.27.1.4.1.3

The MTU for the interface as shown to the higher interface sub-layer (this value should not include the encapsulation or header added by the interface). If IP is operating over the interface, then this value indicates the IP MTU over this interface. For changing the MTU of any interface, the interface must be brought down first - changing MTU while the interface is administratively up is not permitted. If not specified during interface creation, a default value is assigned based on the ifMainType given to the particular interface. While configuring for logical VLAN interfaces, care should be taken to, configure this value as the lowest of the MTU values of the member ports.