Load Balancing & Fault Tolerance
Outbound Load Balancing and Failover (Auto Routing)
running connection is 2:2:2. Auto Routing then has to route both the seventh and eighth connections to WAN3 to make
the ratio 2:2:4 which is 1:1:2. Now in case that the two connections on WAN1 are finished, the number of running
connection becomes 0:2:4. The next two connections must be routed to WAN1 to keep the specified ratio 1:1:2. A
variance that makes this algorithm complexer than Round-Robin is when a connection is finished. In case of failure
happening on WAN2, Auto Routing routes connections among WAN1 and WAN3 with the ratio 1:2.
By Downstream Traffic
Routes connections though the WAN link with lightest downstream traffic load which is the ratio of downstream to the
capibility of a WAN link. Considering the example that WAN1 is 1Mbps and WAN2 is 2Mbps, and downstream traffic of
the both WAN links is 0.5M. Thus the downstream traffic load of WAN1 and WAN2 are 0.5 and 0.25, the next session
will be routed to WAN2.
By Upstream Traffic
Routes connections though the WAN link with lightest upstream traffic load which is the ratio of upstream to the
capibility of a WAN link. Considering the example that WAN1 is 1Mbps and WAN2 is 2Mbps, and upwnstream traffic of
the both WAN links is 0.5M. Thus the upstream traffic load of WAN1 and WAN2 are 0.5 and 0.25, the next session will
be routed to WAN2.
By Total Traffic
Routes connections though the WAN link with lightest traffic load (upstream and downstream) which is the ratio of
total traffic to the capibility of a WAN link. Considering the example that WAN1 is 1Mbps and WAN2 is 2Mbps, and
total traffic of the both WAN links is 0.5M. Thus the traffic load of WAN1 and WAN2 are 0.5 and 0.25, the next session
will be routed to WAN2.
By Optimum Route
Routes sessions through the best-conditioned WAN link based on the evaluation of Optimum Route Detection
(involves the RTT and traffic loading of a WAN link). This algorithm provides real WAN status and avoids the peering
issue between ISPs.
Outbound Load Balancing and Failover (Auto Routing)
Auto Routing Mechanism
Auto Routing load-balances the outbound traffic across multiple WAN links according to a pre-defined routing policies.
During WAN link failures, auto routing will also adjust the routing methods to distribute the outbound traffic ONLY
among the WAN links in fit and working conditions, thus avoiding the failed link(s).
The traditional method of backing up WAN links by having a secondary WAN link taking over the failed link. Basically
having a main line and a second line as backup, aided by any standard router’s backup policy, minimum fault tolerance
can be achieved. This kind of approach means certain lines remain idle for most of the time and it is a waste of
resources. In addition, the router configurations can be tedious.
Another approach for multiple WAN links backup is by dividing the LAN into multiple segments, each doing its own
thing as they are all independent WAN links. Under standard conditions, each segment has its own way using separate
routers. When one of the WAN links fails, the administrator has to change the router configuration to bypass the failed
link. The obvious drawback to this approach is the unnecessary workload for administrators. Whenever WAN link
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FortiWAN Handbook
Fortinet Technologies Inc.
