Technical Tip: Advertising specific subnets via BGP while having aggregate–address settings

[amahdi]

Description	This article describes a scenario where a user is trying to advertise specific subnets 10.90.19.0/30 and 10.90.19.128/25 over BGP while having aggregate address 10.0.0.0/8 configuration. However, it is not advertising the routes to BGP neighbors although the prefix list is configured properly. get router info bgp neighbors 10.90.19.1 advertised-routes % No prefix for neighbor 10.90.19.1 . config router bgp     set as yyyyy     set router-id x.x.x.x     set ebgp-multipath enable         config aggregate-address             edit 1                 set prefix 10.0.0.0 255.0.0.0                 set summary-only enable             next config router prefix-list     edit "NAME"         config rule             edit 1                 set prefix 10.90.19.0 255.255.255.252                 unset ge                 unset le             next             edit 2                 set prefix 10.90.19.128 255.255.255.128                 unset ge                 unset le             next             edit 3                 set action deny                 set prefix any                 unset ge                 unset le             next         end
Scope	FortiGate.
Solution	Disable summary-only settings under aggregate address route to prevent suppressing the child routes to be advertised individually. config router bgp     config aggregate-address         edit 1          set prefix 10.0.0.0 255.0.0.0          set summary-only disable      next end After the BGP soft reset, individual routes were advertised successfully. get router info bgp neighbors 10.90.19.1 advertised-routes VRF 0 BGP table version is 6, local router ID is x.x.x.x Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight RouteTag Path *> 10.90.19.0/30 10.90.19.2 100 32768 0 i <-/-> *> 10.90.19.128/25 10.90.19.2 100 32768 0 i <-/-> In addition to the individual routes, the summary route would also be advertised. In the advertised route when checked, it will be seen that 10.0.0.0/16 will be advertised to the neighbor in addition to 10.90.19.0/30, 10.90.19.128/25.  Related article: Technical Tip: How to implement BGP route summary (aggregation) on a FortiGate