5-2In BGP, the routers that send BGP messages are known as BGP speakers. A BGP speaker receivesand generates new routing information and advertises the information to other BGP speakers. When aBGP speaker receives a route from other AS, if the route is better than the existing routes or the route isnew to the BGP speaker, the BGP speaker advertises the route to all other BGP speakers in the AS itbelongs to.A BGP speaker is known as the peer of another BGP speaker if it exchanges messages with the latter.A group of correlated peers can form a peer group.BGP can operate on a router in one of the following forms.z IBGP (Internal BGP)z EBGP (External BGP)When BGP runs inside an AS, it is called interior BGP (IBGP); when BGP runs among different ASs, it iscalled exterior BGP (EBGP).BGP Message TypeFormat of a BGP packet headerBGP is message-driven. There are five types of BGP packets: Open, Update, Notification, Keepalive,and Route-refresh. They share the same packet header, the format of which is shown by Figure 5-1.Figure 5-1 Packet header format of BGP messagesThe fields in a BGP packet header are described as follows.z Marker: 16 bytes in length. This filed is used for BGP authentication. When no authentication isperformed, all the bits of this field are 1.z Length: 2 bytes in length. This filed indicates the size (in bytes) of a BGP packet, with the packetheader counted in.z Type: 1 byte in length. This field indicates the type of a BGP packet. Its value ranges from 1 to 5,which represent Open, Update, Notification, Keepalive, and Route-refresh packets. Among thesetypes of BGP packets, the first four are defined in RFC1771, and the rest one is defined inRFC2918.OpenOpen massage is used to establish connections between BGP speakers. It is sent when a TCPconnection is just established. Figure 5-2 shows the format of an Open message.
