Technical Tip: Adding second default route with BGP using 'set bestpath-as-path-ignore enable'

[DPadula]

Description	This article describes how to avoid the best AS-PATH criteria while selecting the best route for the default route in the BGP routing protocol allowing us to have two default routes learned via BGP on the routing table
Scope	FortiGate.
Solution	Different from other routing protocols, BGP can only add one route to a destination on the routing table by default, to add more than one route, enable the multipath (ibgp-multipath and/or ebgp-multipath). Sometime enabling multipath will be enough to add routes to the same destination via different neighbours but in some cases we also need to enable bestpath-as-path-ignore. On the network diagram above, both routers have a default route to the Internet. This can be seen on HQ-FW01 FortiGate using the command 'get router info bgp neighbors <neighbour_ip_address> received routes' Using the command 'get router info routing-table bgp' only one default route is shown on the routing table. Observing the BGP configuration, the command 'set ebgp-multipath enable' is already part of it. One default route is showing on the routing table. The reason is the AS-PATH advertised by IPS1 and ISP2 routers. ISP1 router has advertised 0.0.0.0/0 with two AS-PATH (AS 62000 appears twice). That is why FortiGate has added to 0.0.0.0/0 via ISP2 and marked it as best. Adding the command 'set bestpath-as-path-ignore enable' and restarting the BGP peer connection with the command 'execute router clear bgp all' allows BGP to ignore the AS-PATH attribute and add both default routes to the routing table. Note: The command 'get router info bgp network 0.0.0.0' will keep the word best for one of the routes.