t3_mac_set_mtu
int t3_mac_set_mtu(struct cmac *mac, unsigned int mtu);
t3_mac_set_mtu(mac, ifp->if_mtu + ETHER_HDR_LEN
t3_mac_set_mtu(mac, ifp->if_mtu + ETHER_HDR_LEN + ETHER_VLAN_ENCAP_LEN);