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.
GRE tunnel adds 24 byte overhead (4-byte gre header + 20-byte IP header).
|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: )||Tunnel keepalive timeout in seconds. By default keepalive is disabled.|
|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.|
We two sites Site1 with local network range 10.1.101.0/24 and Site2 with local network range 10.1.202.0/24.
First step is to create GRE tunnels. Router on site 1:
/interface gre add name=myGre remote-address=192.168.90.1 local-address=192.168.80.1
Router on site 2:
/interface gre add name=myGre remote-address=192.168.80.1 local-address=192.168.90.1
As you can see tunnel configuration is quite simple.
Now we just need to set up tunnel addresses and proper routing. Router on site 1:
/ip address add address=172.16.1.1/30 interface=myGre /ip route add dst-address=10.1.202.0/24 gateway=172.16.1.2
Router on site 2:
/ip address add address=172.16.1.2/30 interface=myGre /ip route add dst-address=10.1.101.0/24 gateway=172.16.1.1
At this point sites have Layer 3 connectivity over GRE tunnel.