Technical Tip: BGP SD-WAN Route-tag supersedes most specific route

Unlike normal routing behavior, BGP SD-WAN route-tagging prefers the least specific routes which causes more specific routes to be less preferred.

Below is an example wherein the default route is installed by the SD-WAN rule ID 9 due to the said behavior. Notice that 10.56.56.0/24, 10.57.57.0/24, and 10.0.0.0/8 were not included even though these routes are tagged with the same route-tagging as that of the default route.

Another example.

Note that the 10.0.0.0/8 route is used on SD-WAN rule ID 9. Routes to 10.56.56.0/24 and 10.57.57.0/24 were not installed. 

In this example, I have enabled redistribute static route with a route-map and enable capability-default-originate from the Hub. Removing these settings will show the following behavior.

After the changes and BGP restart, the routes to 10.56.56.0/24 and 10.57.57.0/24 were installed properly.

Note: Starting from FortiOS 7.4.7 GA onward, the command 'diagnose sys sdwan route-tag-list' has been removed and replaced with 'diagnose sys route_tag list'.

For more information regarding BGP and SD-WAN route-tagging, see Technical Tip: How to use BGP and SD-WAN for advertising routes and path selection in FortiGate.