Concept Guide

Establish a Session
Information exchange between peers is driven by events and timers. The focus in BGP is on the trac routing policies.
In order to make decisions in its operations with other BGP peers, a BGP process uses a simple nite state machine that consists of six
states: Idle, Connect, Active, OpenSent, OpenConrm, and Established. For each peer-to-peer session, a BGP implementation tracks which
of these six states the session is in. The BGP protocol denes the messages that each peer should exchange in order to change the
session from one state to another.
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.
BGP Attributes for selecting Best Path
Routes learned using BGP have associated properties that are used to determine the best route to a destination when multiple paths exist
to a particular destination.
These properties are referred to as BGP attributes, and an understanding of how BGP attributes inuence route selection is required for the
design of robust networks. This section describes the attributes that BGP uses in the route selection process:
• Weight
• Local Preference
• Multi-Exit Discriminators (MEDs)
• Origin
• AS Path
Border Gateway Protocol (BGP)
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