State DescriptionIdle BGP initializes all resources, refuses all inbound BGP connection attempts, and initiates a TCP connection tothe peer.Connect In this state the router waits for the TCP connection to complete, transitioning to the OpenSent state ifsuccessful.If that transition is not successful, BGP resets the ConnectRetry timer and transitions to the Active statewhen the timer expires.Active The router resets the ConnectRetry timer to zero and returns to the Connect state.OpenSent After successful OpenSent transition, the router sends an Open message and waits for one in return.OpenConfirm After the Open message parameters are agreed between peers, the neighbor relation is established and is inthe OpenConfirm state. This is when the router receives and checks for agreement on the parameters ofopen messages to establish a session.Established Keepalive messages are exchanged next, and after successful receipt, the router is placed in the Establishedstate. Keepalive messages continue to be sent at regular periods (established by the Keepalive timer) toverify connections.After the connection is established, the router can now send/receive Keepalive, Update, and Notification messages to/from its peer.Peer GroupsPeer groups are neighbors grouped according to common routing policies. They enable easier system configuration and managementby allowing groups of routers to share and inherit policies.Peer groups also aid in convergence speed. When a BGP process needs to send the same information to a large number of peers,the BGP process needs to set up a long output queue to get that information to all the proper peers. If the peers are members of apeer group however, the information can be sent to one place and then passed onto the peers within the group.Route ReflectorsRoute reflectors reorganize the iBGP core into a hierarchy and allow some route advertisement rules.NOTE: Do not use route reflectors (RRs) in the forwarding path. In iBGP, hierarchal RRs maintaining forwarding planeRRs could create routing loops.Route reflection divides iBGP peers into two groups: client peers and nonclient peers. A route reflector and its client peers form aroute reflection cluster. Because BGP speakers announce only the best route for a given prefix, route reflector rules are applied afterthe router makes its best path decision.• If a route was received from a nonclient peer, reflect the route to all client peers.• If the route was received from a client peer, reflect the route to all nonclient and all client peers.To illustrate how these rules affect routing, refer to the following illustration and the following steps. Routers B, C, D, E, and G aremembers of the same AS (AS100). These routers are also in the same Route Reflection Cluster, where Router D is the RouteReflector. Router E and H are client peers of Router D; Routers B and C and nonclient peers of Router D.172 Border Gateway Protocol IPv4 (BGPv4)
