5-3Figure 5-2 BGP Open message formatThe fields are described as follows.z Version: BGP version. As for BGP-4, the value is 4.z My Autonomous System: Local AS number. By comparing this filed of both sides, a router candetermine whether the connection between itself and the BGP peer is of EBGP or IBGP.z Hold time: Hold time is to be determined when two BGP speakers negotiate for the connectionbetween them. The Hold times of two BGP peers are the same. A BGP speaker considers theconnection between itself and its BGP peer to be terminated if it receives no Keepalive or Updatemessage from its BGP peer during the hold time.z BGP Identifier: The IP address of a BGP router.z Opt Parm Len: The length of the optional parameters. A value of 0 indicates no optional parameteris used.z Optional Parameters: Optional parameters used for BGP authentication or multi-protocolextensions.UpdateUpdate message is used to exchange routing information among BGP peers. It can propagate areachable route or withdraw multiple pieces of unreachable routes. Figure 5-3 shows the format of anUpdate message.Figure 5-3 BGP Update message formatAn Update message can advertise a group of reachable routes with the same path attribute. Theseroutes are set in the NLRI (network layer reachability information) field. The Path Attributes field carriesthe attributes of these routes, according to which BGP chooses routes. An Update message can alsocarry multiple unreachable routes. The withdrawn routes are set in the Withdrawn Routes field.The fields of an Update message are described as follows.z Unfeasible Routes Length: Length (in bytes) of the unreachable routes field. A value of 0 indicatesthat there is no Withdrawn Routes filed in the message.z Withdrawn Routes: Unreachable route list.
