Troubleshooting Tip: Configuring priority for BGP default routes to establish a primary (preferred) and backup path

[akushwaha]

Description	This article describes the situations where, because of Reverse Path Forwarding (RPF) considerations, the FortiGate requires two default BGP routes across two external interfaces. The setup must satisfy these requirements: incoming traffic is permitted on both interfaces (port1 and port2), while outgoing traffic should prefer port1 exclusively, making ECMP unnecessary.
Scope	FortiGate.
Solution	In the below setup, eBGP is configured between FortiGate and the two ISPs: FortiGate is getting a default route from both ISPs through BGP with the same AD value(20) and Priority (1): The recommended approach is to configure different priorities for both the default routes. The route with the lowest priority value will serve as the best and primary route. With the below configuration, it can be achieved: Configure prefix list: config router prefix-list     edit "Default-Route"         config rule             edit 1                 set prefix 0.0.0.0 0.0.0.0             next         end     next end Configure two Route-maps for both ISPs and set priority : config router route-map     edit "ISP-1"         config rule             edit 1                set match-ip-address "Default-Route"                set set-priority 100     --> Route Priority for ISP-1.             next         end     next end config router route-map    edit "ISP-2"         config rule             edit 1                 set match-ip-address "Default-Route"                 set set-priority 200  --> Route Priority for ISP-2.             next         end     next end Apply the route-map to each BGP neighbor: config router bgp     config neighbor         edit "10.5.139.114"             set route-map-in "ISP-1"         next         edit "10.5.209.34"             set route-map-in "ISP-2"         next     end end In most environments, when setting priority in a BGP route-map, the 'recursive-inherit-priority' setting should also be disabled in the router bgp configuration. This ensures the intended priority assigned in the route-map is not overwritten by the priority of a route used to resolve the next-hop to an egress interface. config router bgp     set recursive-inherit-priority disable end Note: After applying the route-maps, perform a soft reset for BGP neighbors. See the article Technical Tip: BGP soft reset to refresh BGP routing table. Run one of the following commands to soft reset the incoming routes: execute router clear bgp ip A.B.C.D soft in execute router clear bgp ip A.B.C.D in Here, A.B.C.D will be the neighbor's IP address. Now the route will be installed with different priority values: Note: If routes are not visible, the following BGP debugs can be performed for troubleshooting: diagnose ip router bgp all enable diagnose ip router bgp level info diagnose debug enable To stop it: diagnose debug reset