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State Description

Idle BGP initializes all resources, refuses all inbound BGP connection attempts, and initiates a TCP connection to the

peer.

Connect 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 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.

OpenConrm After the Open message parameters are agreed between peers, the neighbor relation is established and is in the

OpenConrm state. This is when the router receives and checks for agreement on the parameters of open

messages to establish a session.

Established 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 Notication messages to/from its peer.

Peer Groups

Peer groups are neighbors grouped according to common routing policies. They enable easier system conguration 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 Reectors

Route reectors reorganize the iBGP core into a hierarchy and allow some route advertisement rules.

NOTE

: Do not use route reectors (RRs) in the forwarding path. In iBGP, hierarchal RRs maintaining forwarding plane RRs could

create routing loops.

Route reection divides iBGP peers into two groups: client peers and nonclient peers. A route reector and its client peers form a route

reection cluster. Because BGP speakers announce only the best route for a given prex, route reector rules are applied after the router

makes its best path decision.

• If a route was received from a nonclient peer, reect the route to all client peers.

• If the route was received from a client peer, reect the route to all nonclient and all client peers.

To illustrate how these rules aect 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 Reection Cluster, where Router D is the Route Reector. 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)

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