BGP Next Hop Address Tracking
By default, every 60 seconds, BGP will scan its next-hop addresses within the BGP table. This scan checks to see that there is a route in the local routing table to reach each one of those next-hop addresses.
If a route is not found for a particular next-hop address, the entry in the BGP table is removed.
BGP next-hop tracking is a feature that reduces the BGP convergence time by monitoring BGP next-hop address changes in the routing table. It’s event-based because it detects changes in the routing table. When it detects a change, it schedules a next-hop scan to adjust the next hop in the BGP table.
After detecting a change, the next-hop scan has a default delay of 5 seconds. Next hop tracking also supports dampening penalties. This increases the delay of the next-hop scan for next-hop addresses that keep changing in the routing table.
It is possible to change the scanning interval of BGP routers for next-hop address tracking. This can be done using the bgp scan-time command. This is applied under the BGP router configuration. The range of values can be between 5 and 60 seconds. Starting with Cisco IOS version 15.X, the minimum configurable scan time has been increased from 5 to 15 seconds.
Links
https://networklessons.com/cisco/ccie-routing-switching-written/bgp-next-hop-address-tracking/
https://forum.networklessons.com/t/bgp-next-hop-address-tracking/3490/34?u=lagapides
https://forum.networklessons.com/t/bgp-next-hop-address-tracking/3490/38?u=lagapides