The attached patch adds a 'preferred' route attribute to check whether the route being processed has been selected as the best-path for the network it belongs to. This attribute does not take in a value, and returns a boolean. This is useful for allowing selection in an add-path enabled BGP session. 

The motivation behind selective add paths is to reduce the number of routes that are shared in an add-path enabled BGP session on a per-prefix basis. Instead of sending all paths to all destinations for an add-path session, we would like to specify prefixes (and/or other route attributes) for which all paths are to be shared. Further, if there are thousands of ways to reach a destination, simply negotiating add paths for a prefix would result in sharing all additional paths to the peer. This in turn would result in a flood of advertisements to be sent between the nodes, and a high memory footprint on the peer. To avoid this, we are considering a filter-based selection criteria that would unlock the potential of purposefully sending additional paths when desired. 

Adding the 'preferred' keyword allows us to always send the best-path whether or not other criteria in the export filter match. In the following example, a route is exported when it is either the best route or one of 172.16.0.0/16, 10.0.0.0/8{16,24} (regardless of best route status). In the case of 8.8.8.0/24, we export it if it's the best route, or if it additionally has the specified extended community attached to the route. 

Cheers,

Trisha