Technical Tip: How to load balance traffic between two interfaces via static routes

nageentaj · ‎04-28-2022

Description

This article describes how to do ECMP load balancing with SD-WAN configuration.

Scope

All FortiOS Models.

Solution

Using SD-WAN load balance of traffic can be done by navigating to SD-WAN -> SDWAN Rules ->Implicit Rule.

There are 5 ways to load balance the traffic as shown below.

The traffic will hit the implicit rule only when traffic does not match any of the rules in the SD-WAN rules or if no rules are configured as shown below.

In a scenario where there are SD-WAN rules that are configured based on certain strategies example Manual, Best quality, Lowest cost (SLA), Maximum Bandwidth (SLA): these strategies help in selecting how the traffic will choose the outgoing interface, but it cannot do load balancing as it does not match the implicit rule.

In this case, create a static route for the same destination with the same administrative distance and priority as shown below, the traffic will match the longest prefix even though there is an SD-WAN interface and load balancing will happen between the two interfaces PORT1 and PORT2.

The routing table will look as shown below.

Here, the '8.8.8.8' prefix length is 32 and the one with the default route is '0', so the table will consider one with a 32 prefix length by default.

Find the active routes for the specific destination.

Note:

config system sdwan

set load-balance-mode measured-volume-based <- Volume-based load-balancing and SD-WAN load-balancing mode settings found under the system sub-menu are solely applicable to the implicit rule. Volume-based load balance is not available in the custom SDWAN rules. To set a load balance strategy in the custom rules use the following commands:

config system sdwan

config service

edit <id>

set load-balance enable

set hash-mode round-robin/source-ip-based/source-dest-ip-

based/inbandwidth/outbandwidth/bibandwidth

end

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