Border Gateway Protocol (BGP) learns routes from a neighbor and flattens the BGP next-hop route into an IGP next-hop route before downloading those routes to the Routing Information Base (RIB). The RIB then downloads those routes into the Forwarding Information Base (FIB) to program the routes in hardware.
FIB compression is enabled for IPv4 and IPv6, supporting up to (approximately) 5.7 M IPv4 routes and 900 K IPv6 routes. Refer to release notes and scale documentation for further information.
Note the following:
FIB next-hop/adjacency comprises the set of IGP paths used for forwarding a packet, for example, ECMP paths used by a route to forward matching packets.
If both a less-specific route and a more-specific route point to the same next-hop/adjacency, the more-specific route is not programmed in hardware.
The packet for the less-specific route hits the parent route with the same next-hop, ensuring that there is no traffic black hole, and forwarding result is the same.
Because compression requires a parent route with the same next-hop, Level 1 routes cannot be compressed, as they may not have a parent route (default route).
Level 1 routes in the Internet BGP FIB can be compressed in the range of 40% to 50%.