Users Guide
path to reach a router external to the AS. EBGP routers exchange information with other EBGP routers and IBGP routers to maintain
connectivity and accessibility.
Classless interdomain routing
BGPv4 supports classless interdomain routing (CIDR) with aggregate routes and AS paths. CIDR denes a network using a prex consisting
of an IP address and mask, resulting in ecient use of the IPv4 address space. Using aggregate routes reduces the size of routing tables.
Path-vector routing
BGP uses a path-vector protocol which 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 ve peers.
Sessions and peers
A BGP session starts with two routers communicating using the BGP protocol. 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 trac routing policies.
Sessions
In operations with other BGP peers, a BGP process uses a simple nite state machine consisting of six states—Idle, Connect, Active,
OpenSent, OpenConrm, and Established. For each peer-to-peer session, a BGP implementation tracks the state of the session. The BGP
protocol denes 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.
OpenConrm 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.
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