Load Balancing & Fault Tolerance
Tunnel Routing
How the Tunnel Routing Works
Here is an example to explain the processes that how Tunnel Routing delivers packets to remote private internal
network via Internet. Here are two FortiWAN sites (FWN-A and FWN-B) connected to Internet with two WAN links
respectively. Two private LAN networks: 192.168.10.0/255.255.255.0 and 192.168.20.0/255.255.255.0 are connected
to FWN-A and FWN-B respectively. Now host 192.168.10.100 would like to communicate with host 192.168.20.100
which is located at remote private LAN. Here are the steps:
1. Host 19.168.10.100 sends the first original packet to FWN-A, source IP and destination IP of the packet are
indicated as 192.168.10.100 and 192.168.20.100.
2. FWN-A's Tunnel Routing takes charge of transferring the packet because it matches a tunnel routing rule (A
routing rule is predefined for packets from 192.168.10.0/255.255.255.0 to 192.168.20.0/255.255.255.0).
3. According the specified balancing algorithm (determining a WAN link for transferring), FWN-A encapsulates the
original packet with GRE and Delivery headers which the source IP and destination IP are indicated as public
addresses 1.1.1.1 (FWN-A's WAN 1) and 3.3.3.3 (FWN-B's WAN 1) respectively.
4. The GRE packet is then transferred via Tunnel 1 (from FWN-A's WAN 1 to FWN-B's WAN 1 via Internet).
5. FWN-B receives this GRE packet and decapsulates it to recover the original packet.
6. The original packet then is forwarded to host 192.168.20.100 in the private LAN network.
7. The subsequent packets (for example the packet 2 in the figure below) of the session from host 192.168.10.100
are transferred in the same way except the different tunnels that balancing algorithm determines.
After the basic concept how Tunnel Routing transfers packets, several topics related to Tunnel Routing are explained
in detail.
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FortiWAN Handbook
Fortinet Technologies Inc.
