This reports the total number of forwarding prefixes currently held in the CEF Forwarding Information Base, FIB, for whichever IP version, v4 or v6, is specified by the paired cefFIBIpVersion index object. This is essentially the live route count CEF is actually using to forward traffic, so a sudden drop in this number on a router that should be receiving a full routing table is a strong, specific signal of a routing protocol session drop or route-filtering issue upstream, rather than a general CEF health question. A core router carrying the full internet routing table might show this object at roughly 900,000 IPv4 prefixes; a sudden drop to a few hundred would indicate the router lost its BGP peering and fell back to a nearly-empty FIB.