Troubleshooting Tip: Prepended BGP routes not installed into the routing table

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Description	This article describes how to avoid having BGP routes received and filtered out without any route-map or prefix-list applied.
Scope	FortiGate, SD-WAN.
Solution	In Multipath communication between 2 sites with routing done via BGP, it is common to use BGP prepending mechanism to define path preferences. There are 2 types of BGP neighborship: EBGP (Exterior BGP): The AS (Autonomous System) of peers is different and should be used for route exchanges between 2 different organizations for a route exchange via the internet. IBGP (Interior BGP): The AS of neighbors is the same and should be used to exchange routes between devices of the same organization, for a route exchange via an intranet network. The prepending mechanism must be used on EBGP (Exterior BGP), not on Internal IBGP (Interior BGP). The related RFC document describes the scenario of routes prepended over the Internet, so over EBGP neighborships. Prepending applied on IBGP is a not covered scenario.   Here an example of Wrong configuration prepending over iBGP and what’s happens using it: The route: 10.50.20.0/24, prepended is sent over an IBGP neighborship (as confirmed from the 'i' at the left of the subnet, on the first command output of the last screenshot). 172.100.0.2 is the IP of the first peer, the route advertiser, and 172.100.0.1 IP of the second peer, the route receiver. As visible from the second peer configuration:   There are not route-maps or prefix-lists applied to it, to filter out the route. On the second peer, the route 10.50.20.0/24 is received and discharged: More details about the 2 commands are here: How to check BGP advertised and received routes Debugging the BGP process as explained here: Check how BGP traffics flow with debugs  The following message appears: ----- 2024-08-21 14:09:13 BGP: 172.100.0.2-Outgoing [RIB] Update: Prefix 10.50.20.0/24 path_id 0 denied due to as-path contains our own AS ----- The route is correctly discharged for loop prevention mechanism and prepending over an IBGP neighborship is not supported. Suggestions about the correct configuration to use: To influence route selection behavior with IBGP, is possible to use other BGP attributes, likes: Local Preferences: BGP conditional advertisement example in a BGP  Weights: Use BGP Weight attribute to prefer default route  Other suggestions are in chapter 4 of the RFC document quoted above. Important notes: Into a Fortinet SD-WAN environment: Implement BGP communities and associated route tags is the correct way to do route selection, here is a configuration example: How to use BGP and SD-WAN for advertising routes  To use the ADVPN feature over BGP, is necessary to use IBGP, as indicated here: ADVPN with BGP as the routing protocol Related articles: How to configure BGP AS prepending  BGP route selection-process