6-2z Reducing bandwidth consumption by advertising only incremental updates and thereforeapplicable to advertising a great amount of routing information on the Internetz Eliminating routing loops completely by adding AS path information to BGP routesz Providing abundant policies to implement flexible route filtering and selectionz Good scalabilityA router advertising BGP messages is called a BGP speaker. It establishes peer relationships withother BGP speakers to exchange routing information. When a BGP speaker receives a new route or aroute better than the current one from another AS, it will advertise the route to all the other BGP peersin the local AS.To simplify configuration, multiple peers having an identical policy can be organized as a peer group.BGP runs on a router in either of the following two modes:z iBGP (internal BGP)z eBGP (external BGP)BGP is called iBGP when it runs within an AS and is called eBGP when it runs between ASs.Formats of BGP MessagesHeaderBGP has five types of messages:z Openz Updatez Notificationz Keep-alivez Route-refreshThey have the same header, as shown below:Figure 6-1 BGP message headerz Marker: The 16-byte field is used to delimit BGP messages. The Marker must be all ones.z Length: The 2-byte unsigned integer indicates the total length of the message.
