GRE RFC 1701
GRE (generic routing encapsulation) is a tunneling protocol that was originally developed by Cisco. It can encapsulate wide variety of protocols creating virtual point-to-point link.
GRE the same as IPIP and EoIP were originally developed as stateless tunnels. Meaning that if remote end of the tunnels goes down all traffic that was routed over the tunnels gets blackholed. To solve this problem RouterOS have added keepalive feature for GRE tunnels.
|arp (disabled | enabled | proxy-arp | reply-only; Default: )||Address Resolution Protocol mode|
|comment (string; Default: )||Short description of the tunnel.|
|disabled (yes | no; Default: no)||Whether tunnel is enabled.|
|keepalive (integer [1..4294967295]; Default: )|
|l2mtu (integer [0..65536]; Default: 65535)||Layer2 Maximum transmission unit.|
|local-address (IP; Default: 0.0.0.0)||Ip addres that will be used as local tunnel end. If set to 0.0.0.0 then ip address of outgoing interface will be taken.|
|mtu (integer [0..65536]; Default: 1476)||Layer3 Maximum transmission unit.|
|name (string; Default: )||Name of the tunnel.|
|remote-address (IP; Default: )||IP address of remote tunnel end.|