826 CHAPTER 56: BGP CONFIGURATIONA router advertising BGP messages is called a BGP speaker, which exchanges newrouting information with other BGP speakers. When a BGP speaker receives a newroute or a route better than the current one from another AS, it will advertise theroute to all the other BGP speakers in the local AS.BGP speakers call each other peers, and several associated peers form a peergroup.BGP runs on a router in one of the following two modes:■ IBGP (Interior BGP)■ EBGP (External BGP)BGP is called IBGP when it runs within an AS and is called EBGP when it runsbetween ASs.Formats of BGPMessagesHeaderBGP message involves five types:■ Open message■ Update message■ Notification message■ Keep-alive message■ Route-refresh messageThey have the same header, as shown below:Figure 231 BGP message header■ Marker: The 16-octet field is used for BGP authentication calculation. If noauthentication information is available, then the Marker must be all ones.■ Length: The 2-octet unsigned integer indicates the total length of the message.■ Type: This 1-octet unsigned integer indicates the type code of the message. Thefollowing type codes are defined: 1-Open 2-Update 3-Notification 4-Keepalive5-Route-refresh. The former four are defined in RFC1771, the latter onedefined in RFC2918.MarkerLength Type2 bytes 1 bytes16 bytes
