This is a function enabled by default for IOS currently supported. To disable, configure the no bgp nexthop trigger enable command.
By default, the scan will be performed five seconds after the route is updated. This value can be adjusted by the bgp nexthop trigger delay <SEC> command. If there is no route which can reach to the next-hop address when performing the scan, all the BGP routes that use the next-hop address will be deleted.
The address targeted for tracking is the next-hop address of the BGP route. You can check this address in the Next Hop column using the show ip bgp command. If many prefixes exist, you can check the summarized address using the show ip bgp nexthops command.
When a link down occurs between R2-R4, the route to 220.127.116.11. will be lost, which triggers the scan of next-hop address. The following debug is an example of output during scan. You can see that the scan is scheduled after the route is deleted, and is performed 5 seconds later. This next-hop scan deletes the route whose next-hop is 18.104.22.168.
NHT also provides a dampening function. For each address updated, 500 is added to the penalty. When the next-hop scan is to be performed is determined by the penalty value after addition.
950 or less: Scheduled as configured, in seconds (default is five seconds)
More than 950: Scheduled for when the penalty value decreases to 100 or less by the calculation formula described below
The penalty value will be added per update event, but if the next-hop scan has already been scheduled, it will not be rescheduled.
Penalty values are not simply subtracted from, but are decreased by half every eight seconds (i.e. they have a half-life of eight seconds). The precise calculation formula is complex, but the value decreases to approximately 92% every second. For example, if 500 is added and there is no other event, the penalty value will decrease to 100 or less after 19 seconds. Unlike ip dampening, this parameter (added penalty value, half-life value) cannot be modified. Also, when NHT is used, this dampening operation cannot be stopped.
Since the target of the penalty value addition is a general update operation, which includes any addition, deletion, and update of routes, the penalty value will be added every time such an event occurs. Note that addition occurs per address regardless of the number of routes that use the next-hop. In this configuration example, next-hop 22.214.171.124 is used by two prefix, but this does not mean that the penalty value will be doubled.
Output example when E0/0 of R1 is down and three routes, 126.96.36.199/32, 188.8.131.52/32, and 184.108.40.206/32, are updated
This dampening does not have much effect if IGP has converged at the time of the next-hop scan immediately after a route change event occurred. However, it may need to be investigated when a flap frequently occurs on the routes that next-hop uses.
The following is an example of intentionally causing successive flaps to 220.127.116.11/32.