BGP Convergence and Updates

BGP scanner process monitors the next hop of installed routes to verify next-hop reachability. It is also responsible to select, install, and validate the BGP best path. By default, the BGP scanner is used to poll the RIB for this information every 60 seconds. During the 60 second time period between scan cycles, Interior Gateway Protocol (IGP) instability or other network failures can cause black holes and routing loops to temporarily form.
BGP scan process is also responsible for the checks to determine whether the conditional advertisement should or should not advertise the conditional route. It also checks whether route dampening information needs to be updated.

bgp scan-time

There is also a VPN4 equivalent that is configured under the VPN4 address family and the syntax is slightly different. By default it runs every 15 seconds.

bgp scan-time import

Ref:
http://21500.net/?tag=bgp-scan-interval
http://www.networkers-online.com/blog/2008/12/bgp-performance-tunning-convergence-stability-scalability-and-nsf-part-2/
http://routing-bits.com/2009/08/07/bgp-convergence-and-updates/