Description
This article describes the Equal Cost Multi-Path (ECMP), which is a mechanism that allows a FortiGate to load-balance routed traffic over multiple gateways.
Just like routes in a routing table, ECMP is considered after policy routing, so any matching policy routes will take precedence over ECMP. When ECMP paths are exceeded, this can cause problems with all routing when a new route is added into the ECMP path.
Scope
All Fortigate devices running FortiOS version 6.0 and above.
Solution
ECMP prerequisites are as follows:
- Routes must have the same destination and costs. In the case of static routes, costs include distance and priority.
- Routes are sourced from the same routing protocol. Supported protocols include static routing, OSPF, and BGP.
The default setting for the number of max ECMP paths allowed by a FortiGate is 255.
config system settings
set ecmp-max-paths <number of paths> <----- Default is 255.
end
The default setting of 255 is also the maximum value that can be configured. This is a hard limit and can not be increased. FortiGate does not support more than 255 ECMP paths.
Setting ecmp-max-paths to the lowest value of 1 is equivalent to disabling ECMP.
To configure the ECMP algorithm from the CLI:
config system settings
set v4-ecmp-mode {source-ip-based* | weight-based | usage-based | source-dest-ip-based}
end
ECMP mode can be adjusted for each VDOM.
The following table summarizes the different load-balancing algorithms supported by each:
Related articles:
Technical Tip: ECMP – Load balancing algorithms for IPv4 and IPv6
