Sessions And Peers; Establish A Session - Dell C9000 Series Networking Configuration Manual

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Sessions and Peers

When two routers communicate using the BGP protocol, a BGP session is started. The two end-points of that

session are Peers. A Peer is also called a Neighbor.

Information exchange between peers is driven by events and timers. The focus in BGP is on the traffic routing

In order to make decisions in its operations with other BGP peers, a BGP process uses a simple finite state

machine that consists of six states: Idle, Connect, Active, OpenSent, OpenConfirm, and Established. For each

peer-to-peer session, a BGP implementation tracks which of these six states the session is in. The BGP

protocol defines the messages that each peer should exchange in order to change the session from one state

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.

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 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

Border Gateway Protocol IPv4 (BGPv4)