Which two features improve BGP convergence?

The BGP Support for Next-Hop Address Tracking feature is enabled by default when a supporting
Cisco software image is installed. BGP next-hop address tracking is event driven. BGP prefixes
are automatically tracked as peering sessions are established. Next-hop changes are rapidly
reported to the BGP routing process as they are updated in the RIB. This optimization improves
overall BGP convergence by reducing the response time to next-hop changes for routes installed
in the RIB. When a best path calculation is run in between BGP scanner cycles, only next-hop
changes are tracked and processed.
BGP routers and route reflectors (RRs) propagate only their best path over their sessions. The
advertisement of a prefix replaces the previous announcement of that prefix (this behavior is
known as an implicit withdraw). The implicit withdraw can achieve better scaling, but at the cost of
path diversity.
Path hiding can prevent efficient use of BGP multipath, prevent hitless planned maintenance, and
can lead to MED oscillations and suboptimal hot-potato routing. Upon nexthop failures, path hiding
also inhibits fast and local recovery because the network has to wait for BGP control plane
convergence to restore traffic. The BGP Additional Paths feature provides a generic way of
offering path diversity; the Best External or Best Internal features offer path diversity only in limited
scenarios.

The BGP Additional Paths feature provides a way for multiple paths for the same prefix to be
advertised without the new paths implicitly replacing the previous paths. Thus, path diversity is
achieved instead of path hiding.
References: http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_bgp/configuration/15-1sg/irgnexthop-track.html
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/irg-xe-3sbook/bgp_additional_paths.html