GRE RFC 1701
GRE (Generic Routing Encapsulation) is a tunnelling protocol that was originally developed by Cisco. It can encapsulate a wide variety of protocols creating a virtual point-to-point link.
GRE is the same as IPIP and EoIP which were originally developed as stateless tunnels. Which means that if the remote end of the tunnel goes down, all traffic that was routed over the tunnels will gets blackholed. To solve this problem, RouterOS have added 'keepalive' feature for GRE tunnels.
GRE tunnel adds a 24 byte overhead (4-byte gre header + 20-byte IP header).
|clamp-tcp-mss (yes | no; Default: yes)|
|comment (string; Default: )||Short description of the tunnel.|
|disabled (yes | no; Default: no)||Enables/disables tunnel.|
|dont-fragment (inherit | no; Default: no)|
|dscp (inherit | integer [0-63]; Default: )||Set dscp value in Gre header to a fixed value or inherit from dscp value taken from tunnelled traffic|
|ipsec-secret (string; Default: )||When secret is specified, router adds dynamic ipsec peer to remote-address with pre-shared key and policy with default values (by default phase2 uses sha1/aes128cbc). Both local-address and remote-address of the tunnel must be specified for router to create valid ipsec policy.|
|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 address that will be used for local tunnel end. If set to 0.0.0.0 then IP address of outgoing interface will be used.|
|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 have 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 both sites have Layer 3 connectivity over GRE tunnel.