The Type field indicates the type of message being sent. There are four pos- sible values, as shown in Table 7.1.
Let's look at the different message types.
This is the first message sent after a TCP session has been established between one or more peers. This message is used to identify the AS that the router is a member of, to agree on protocol parameters, and to determine the protocol timers the session will use. Figure 7.8 shows the additional fields included in the BGP header for an OPEN message.
F I G U R E 7 . 8
The additional fields added to the BGP common header for an OPEN message type
Let's take a look at each of the additional field types added to the OPEN