Hello Narad
The BGP hold timer is used for neighbor adjacency states, and not for the validity of particular routes in the BGP table. This hold timer, which indeed is initially set to 180 seconds, is involved in maintaining BGP peerings. Every time a keepalive or update message is received, the hold timer is reset. If the hold timer is exhausted without any messages, then the BGP neighbor peering is torn down. More info on this process can be found in the following lesson:
The concept of a hold-down timer is something used by IGPs. BGP does not use such a timer for the validity of BGP routes. Instead, it performs a scan every 60 seconds. It scans all of the next-hop IPs found in its BGP table to see if they are reachable. Specifically, it checks to see if the local routing table has a route that serves those next hop IPs.
Now under normal circumstances, this scan will take place every 60 seconds. So if the route to a particular next hop of a BGP route is removed from the local routing table, it may take up to 60 seconds for the BGP process to realize it.
Next hop address tracking, as stated in the lesson, is event-based. This means that if there is a change in the routing table, BGP is instructed to schedule a next-hop scan in 5 seconds. The process is then further described in the lesson.
I hope this has been helpful!
Laz