Users Guide

BGP uses a path-vector protocol that maintains dynamically updated path information. Path information updates which return
to the originating node are detected and discarded. BGP does not use a traditional Internal Gateway Protocol (IGP) matrix but
makes routing decisions based on path, network policies, and/or rule sets.
Full-mesh topology
In an AS, a BGP network must be in full mesh for routes received from an internal BGP peer to send to another IBGP peer.
Each BGP router talks to all other BGP routers in a session. For example, in an AS with four BGP routers, each router has three
peers; in an AS with six routers, each router has five peers.
Sessions and peers
A BGP session starts with two routers communicating using the BGP. The two end-points of the session are called peers. A peer
is also called a neighbor. Events and timers determine the information exchange between peers. BGP focuses on traffic routing
policies.
Sessions
In operations with other BGP peers, a BGP process uses a simple finite state machine consisting of six states—Idle,
Connect, Active, OpenSent, OpenConfirm, and Established. For each peer-to-peer session, a BGP implementation
tracks the state of the session. The BGP defines the messages that each peer exchanges to change the session from one state
to another.
Idle BGP initializes all resources, refuses all inbound BGP connection attempts, and starts a TCP connection
to the peer.
Connect Router waits for the TCP connection to complete and transitions 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 Router resets the ConnectRetry timer to zero and returns to the Connect state.
OpenSent Router sends an Open message and waits for one in return after a successful OpenSent transition.
OpenConfirm Neighbor relation establishes and is in the OpenConfirm state after the Open message parameters are
agreed on between peers. The router then receives and checks for agreement on the parameters of the
open messages to establish a session.
Established Keepalive messages exchange, and after a successful receipt, the router is in the Established state.
Keepalive messages continue to send at regular periods. The keepalive timer establishes the state to
verify connections.
After the connection is established, the router sends and receives keepalive, update, and notification messages to and from its
peer.
Peer templates
Peer templates allow BGP neighbors to inherit the same outbound policies. Instead of manually configuring each neighbor with
the same policy, you can create a peer group with a shared policy that applies to individual peers. A peer template provides
efficient update calculation with a simplified configuration.
Peer templates also aid in convergence speed. When a BGP process sends the same information to many peers, a long output
queue may be set up to distribute the information. For peers that are members of a peer template, the information is sent to one
place then passed on to the peers within the template.
Martian addresses
Martian addresses are invalid networks on the Internet.
Martian addresses are special IPv4 and IPv6 addresses which are not routed by routing devices on the Internet. OS10 considers
the following as Martian prefixes:
● 0.0.0.0/8
● 127.0.0.0/8
● 224.0.0.0/4
● :: / 128
Layer 3
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