Explanation/Reference:
http://www.networkers-online.com/blog/2008/12/bgp-performance-tunning-convergence-stability- scalability-and-nsf-part-2/ Background BGP scanner
It is responsible for BGP housekeeping by scanning both the BGP RIB and the IP RIB and cleaning and sorting things out.
BGP monitors the next hop of the installed routes to verify next-hop reachability and to select, install, and validate the BGP best path. By default, the BGP scanner polls the RIB for this information every 60 seconds. During the 60 second time period between scan cycles, IGP instabilities or other network failures can cause temporarily black holes and routing loops.
NOTE With Cisco IOS the default timer is 60 seconds for the IPv4 address family and 15 seconds for the VPNv4 address family in order to optimize the VPNs routing table convergence.
This timer can be controlled via the following command:
Router(config-router)#bgp scan-time <5-60>