What is difference between Hard Reset and soft reset in BGP ?

In the context of BGP (Border Gateway Protocol), the terms "hard reset" and "soft reset" refer to different ways of resetting or restarting BGP peering sessions with neighboring routers. Here are the differences between the two:

Hard Reset:

A hard reset in BGP involves completely tearing down the BGP peering session with a neighboring router and then reestablishing it from scratch. When a hard reset is performed, all BGP routes and routing information exchanged with the neighbor are lost. The BGP session is terminated, and a new session is initiated, requiring the routers to exchange BGP updates and reestablish their routing tables. A hard reset is typically initiated manually or by explicitly clearing the BGP session. It is a more disruptive method and can lead to temporary routing instability during the reset process.

Soft Reset:

A soft reset in BGP is a less disruptive method of resetting the BGP peering session. Instead of tearing down the entire session, a soft reset involves refreshing the BGP session and re-advertising the existing BGP routes to the neighbor. The router sends a request to the neighbor to retransmit its BGP routes, and the neighbor responds by re-sending the routes it previously advertised. This allows the router to update its local routing table without completely restarting the BGP session. A soft reset is typically initiated to refresh the routing information or to trigger a synchronization of BGP routes between the routers without causing major disruptions to the network.

In summary, a hard reset in BGP involves terminating and reestablishing the BGP peering session, resulting in a complete loss of routing information. A soft reset, on the other hand, refreshes the BGP session by re-advertising the existing routes, allowing for an update of the routing table without tearing down the session. The choice between a hard reset and a soft reset depends on the specific requirements and objectives of the network administrator or operator.

Basis	Hard Reset	Soft Reset
Goal	The TCP session with the BGP neighbour is killed by Hard Reset, and it must be restarted. It terminates the TCP connection between the peers, re-establishes the connection with a BGP open message, and resumes normal peer-to-peer message exchanges.	There are two types of soft resets: 1. Soft reset with Route refresh: A soft reset with Route refresh allows BGP peers to seek an update without destroying the neighbour relationship. 2. Soft-reconfiguration: Because the route-refresh request is not issued to the BGP neighbour, soft-reconfiguration stores a copy of BGP routes sent in the BGP update from our peer.
Direction	The direction of connection in hard reset can be both in and out.	The direction of connection in soft reset is either in or out.
Utilization of memory	It does not necessitate any additional RAM to store another table.	It necessitates more RAM because the router now stores two BGP tables for each neighbour instead of one.
Impact	The hard reset has the potential to drastically impair network connectivity	Only those prefixes affected by the policy change will be affected by the soft reset.
Use Case	When a soft reset fails to resolve the problem, a hard reset should be utilised as a final resort.	The soft technique is the most popular and widely employed.
Prerequisite	There are no requirements for this activity.	In soft reset with the Route refresh functionality, both peers may be required to provide the Route Refresh capability.
Rate Of Convergence	Because processing the entire table takes a lengthy time, it necessitates an extremely long convergence time.	Its convergence time is significantly less than that of a hard reset.
Commands Used	clear ip bgp {* | neighbor ip | peer-group}	clear ip bgp {neighbor ip} soft out clear ip bgp {neighbor ip} soft in