It is Used to report

BGP error. It results in a reset of neighbor relationship.

It is Sent periodically to maintain the neighbor relationship. If the

Keepalive message is not received within the negotiated Hold timer than BGP

neighborship will be turned down.

It exchanges Path Attributes and the associated prefix/length (NLRI)

that use those attributes.

It is Used to establish a neighbor relationship and exchange parameters,

including autonomous system number and authentication values.