262
Chapter 7
BGP's Basic Components
Type
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.
OPEN Message
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
message type.
T A B L E 7 . 1
Values for the Type Field
Type Value
Message Type
1
OPEN message
2
UPDATE message
3
NOTIFICATION message
4
KEEPALIVE message
Common
Header
Version
(1 octet)
Hold
Time
(2 octets)
BGP
Identifier
(4 octets)
My
Autonomous
System
(2 octets)
Optional
Paramters
Length
(1 octet)
Optional
Paramters
(length
varies)
+
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com