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)||Controls whether to change MSS size for received TCP SYN packets. When enabled, a router will change the MSS size for received TCP SYN packets if the current MSS size exceeds the tunnel interface MTU (taking into account the TCP/IP overhead).The received encapsulated packet will still contain the original MSS, and only after decapsulation the MSS is changed.|
|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).|
|keepalive (integer[/time],integer 0..4294967295; Default: 10s,10)||Tunnel keepalive parameter sets the time interval in which the tunnel running flag will remain even if the remote end of tunnel goes down. If configured time,retries fail, interface running flag is removed.
Parameters are written in following format:
|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.