Dell MX5108n OS10 Enterprise Edition User Guide for PowerEdge MX IO Modules Re - Page 273

Classless interdomain routing BGPv4 supports classless interdomain routing (CIDR) with aggregate routes and AS paths. CIDR defines a network using a prefix consisting of an IP address and mask, resulting in efficient 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 five 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 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 protocol defines the messages that each peer exchanges to change the session from one state to another. Idle Connect Active OpenSent OpenConfirm Established BGP initializes all resources, refuses all inbound BGP connection attempts, and starts a TCP connection to the peer. 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. Router resets the ConnectRetry timer to zero and returns to the Connect state. Router sends an Open message and waits for one in return after a successful OpenSent transition. 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. 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. Layer 3 273