Technical Tip: Blackhole route for BGP Summarization

[Serxhio]

Description	This article describes a BGP route summarization technique.
Scope	FortiGate.
Solution	This article considers the following connected networks: 10.174.0.0/20. 10.163.0.0/20. The purpose is to advertise networks via a summary route. In Border Gateway Protocol (BGP), network advertisements are typically derived from entries in the local Routing Information Base (RIB). By default, BGP only advertises prefixes that are present in the routing table. This ensures that only valid and reachable routes are propagated to BGP peers, maintaining route integrity across the network. However, in certain scenarios (such as route summarization, manual aggregation, or lab testing), it may be necessary for BGP to advertise prefixes that are not explicitly present in the RIB. When the network-import-check option is disabled, BGP changes its default behavior: BGP will advertise configured network statements even if the specified prefixes do not exist in the local routing table. This allows advertisement of summary routes or aggregated prefixes without requiring an exact match in the RIB. To advertise the networks by a single summary route, a matching blackhole route can be added to the routing table. config router static     edit 2         set dst 10.160.0.0 255.240.0.0         set distance 254         set blackhole enable     next  end The entry for 10.160.0.0/20 is now in the RIB. Create a prefix list: config router prefix-list     edit "blackhole_summary"         config rule             edit 1                 set prefix 10.160.0.0 255.240.0.0                 unset ge                 unset le             next         end     next end Put the prefix list in a route map: config router route-map     edit "B_S"         config rule             edit 1                 set match-ip-address "blackhole_summary"             next         end     next end Redistribute the static routes on BGP by filtering on the route map created: config router bgp     config redistribute "static"         set status enable         set route-map "B_S"     end end Result: Related articles: Technical Tip: How to implement BGP route summary (aggregation) on a FortiGate  Creating a blackhole route - FortiGate documentation