Users Guide

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.
OpenConfirm 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.
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 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.
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 influence 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
● Next Hop
NOTE:
There are no hard coded limits on the number of attributes that are supported in the BGP. Taking into account
other constraints such as the Packet Size, maximum number of attributes are supported in BGP.
Communities
BGP communities are sets of routes with one or more common attributes. Communities are a way to assign common attributes
to multiple routes at the same time.
NOTE: Duplicate communities are not rejected.
168 Border Gateway Protocol (BGP)