BGP Fundamentals
BGP Operating Modes
BGP is called Internal BGP (IBGP) when it runs within an AS; it is called External BGP (EBGP) when it runs between ASs, as shown in Figure 9-2.
Figure 9-2 BGP operating modes
Roles in Transmitting BGP Messages
Speaker: Any router that sends BGP messages is called a BGP speaker. The speaker receives or generates new routing information and then advertises the routing information to other BGP speakers. After receiving a route from another AS, a BGP speaker compares the route with its local routes. If the route is better than its local routes, or the route is new, the speaker advertises this route to all other BGP speakers.
Peer: BGP speakers that exchange messages with each other are called peers.
BGP Messages
BGP runs by sending five types of messages: Open, Update, Notification, Keepalive, and Route-refresh.
Open: The first message sent after a TCP connection is set up is an Open message, which is used to set up BGP peer relationships. After a peer receives an Open message and the peer negotiation is successful, the peer sends a Keepalive message to confirm and maintain the peer relationship. Then, peers can exchange Update, Notification, Keepalive, and Route-refresh messages.
Update: This type of message is used to exchange routes between BGP peers.
An Update message can advertise multiple reachable routes with the same attributes. These route attributes are applicable to all destination addresses (expressed by IP prefixes) in the Network Layer Reachability Information (NLRI) field of the Update message.
An Update message can be used to delete multiple unreachable routes. Each route is identified by its destination address (using the IP prefix), which identifies the routes previously advertised between BGP speakers.
An Update message can be used only to delete routes. In this case, it does not need to carry the route attributes or NLRI. In addition, an Update message can be used only to advertise reachable routes. In this case, it does not need to carry information about the deleted routes.
Notification: When BGP detects an error, it sends a Notification message to its peer. The BGP connection is then torn down immediately.
Keepalive: BGP periodically sends Keepalive messages to peers to maintain peer relationships.
Route-refresh: This type of message is used to request that the peer resend all reachable routes.
If all BGP routers are enabled with the Route-refresh capability and the import policy of BGP changes, the local BGP router sends a Route-refresh message to its peers. After receiving the Route-refresh message, the peers resend their routing information to the local BGP router. In this manner, BGP routing tables are dynamically refreshed and new routing policies are used without tearing down BGP connections.
BGP Finite State Machine
The BGP Finite State Machine (FSM) has six states: Idle, Connect, Active, OpenSent, OpenConfirm, and Established.
Three common states during the establishme
