Which BGP message type is used to verify connectivity during and after BGP session establishment?

The BGP KEEPALIVE message is used to maintain a session between BGP peers. After a session is established, peers periodically exchange KEEPALIVE messages to confirm that the connection is still active. If a router does not receive a KEEPALIVE message from its peer within the negotiated Hold Time interval, it considers the peer down and terminates the BGP session. This mechanism serves as the primary method for verifying connectivity throughout the life of the BGP session.

A. Hello messages are used by Interior Gateway Protocols (IGPs) like OSPF and IS-IS for neighbor discovery and maintenance, not by BGP.
B. UPDATE messages are used to advertise, modify, or withdraw routing information (NLRI) between BGP peers, not for connectivity verification.
C. The OPEN message is used only once at the beginning of a session to negotiate parameters like AS number and Hold Time.

1. K. Sriram, et al., "RFC 8212: Default BGP KEEPALIVE Timer Behavior," IETF, July 2017, Section 1. "BGP speakers exchange KEEPALIVE messages to ensure that the connection between them is alive."

2. Y. Rekhter, T. Li, S. Hares, "RFC 4271: A Border Gateway Protocol 4 (BGP-4)," IETF, January 2006, Section 4.4. "BGP does not use any TCP-based keep-alive mechanism... Instead, KEEPALIVE messages are exchanged between peers often enough not to cause the Hold Timer to expire."

3. Nokia, "7750 SR OS Router Configuration Guide," Release 22.10.R1, Part No: 3HE 18149 AAAD TQZZA, 2022, BGP Section. The guide describes the function of keepalive and hold-time timers for maintaining BGP sessions.