Dvmrp tunneling control messages encapsulated type.
noencaps is the mode that control messages are sent in regular
igmp packets with ip protocol type set to IP_PROTOCOL_IGMP.
encaps is the mode that control messages are encapsed inside
ip packets with ip protocol type set to IP_PROTOCOL_IPINIP.
All DVMRP messages sent across tunnels will be encapsulated
in the same way as data packets in this mode.