Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.1P1 - Page 170

State Idle Connect Description BGP initializes all resources, refuses all inbound BGP connection attempts, and initiates a TCP connection to the peer. In this state the router waits for the TCP connection to complete, transitioning to the OpenSent state if successful. If that transition is not successful, BGP resets the ConnectRetry timer and transitions to the Active state when the timer expires. Active OpenSent OpenConfirm Established The router resets the ConnectRetry timer to zero and returns to the Connect state. After successful OpenSent transition, the router sends an Open message and waits for one in return. After the Open message parameters are agreed between peers, the neighbor relation is established and is in the OpenConfirm state. This is when the router receives and checks for agreement on the parameters of open messages to establish a session. Keepalive messages are exchanged next, and after successful receipt, the router is placed in the Established state. Keepalive messages continue to be sent at regular periods (established by the Keepalive timer) to verify connections. After the connection is established, the router can now send/receive Keepalive, Update, and Notification messages to/from its peer. Peer Groups Peer groups are neighbors grouped according to common routing policies. They enable easier system configuration and management by 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 a peer group however, the information can be sent to one place and then passed onto the peers within the group. Route Reflectors Route 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 plane RRs 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 a route reflection cluster. Because BGP speakers announce only the best route for a given prefix, route reflector rules are applied after the 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 are members of the same AS (AS100). These routers are also in the same Route Reflection Cluster, where Router D is the Route Reflector. Router E and H are client peers of Router D; Routers B and C and nonclient peers of Router D. 170 Border Gateway Protocol IPv4 (BGPv4)