Technical Tip: 'set-community-additive' in BGP for optimizing route-maps

[nevan]

Description	This article describes how to use set-community-additive in BGP route-map and the purpose of using it.
Scope	FortiGate.
Solution	In FortiGate BGP, the set-community-additive enable command ensures that new BGP community values will be added to the existing ones rather than replacing them.  By default, when using a set community in a route-map, any previously assigned community values are overwritten. Enabling set-community-additive allows the new communities to be appended while keeping the original ones intact. Here is the configuration example from the CLI session. CLI: config router route-map        edit "route-map"              config rule                     edit 1                         set action permit                         set match-ip-address "prefix-list"                         set match-community 65001:100 65001:101                         set set-community-additive enable                     next               end        next end The following behavior will be expected from the above configuration: This route-map will be applied to routes matching 'prefix-list'. It will assign BGP communities 65001:100 and 65001:101 both. The 'set-community-additive' enable option ensures that these values are added rather than replacing any existing ones. This setting is useful when users want to preserve existing community values while adding new ones, helping to maintain proper routing policies and traffic control across different networks. Related article: Technical Note : Adding BGP community string to route updates and control BGP routes advertisements ...