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Chapter 2

Protocol Attributes

You can also accomplish the second prerequisite in one of three ways:

BGP learns of networks that it needs to advertise through other BGP adver-
tisements, network statements, and redistribution of an IGP into BGP. The
last option is not recommended because of the instability incurred--redistri-
bution can cause routing loops and route flapping.

BGP Packet Structure

The BGP header is only 20 bytes long and contains three fields. The first field
is 16 bytes long and is called the marker field. This field contains a value
that can be predicted by the receiving system. It is used for authentication
purposes.

The next field is the length field and it is two bytes long. The length

field contains the size of the entire message, in bytes. Finally, the type
field

is the last field, and it is two bytes in length. It is used to indicate the

message type. Figure 2.19 gives a graphical representation of a BGP packet
header.

F I G U R E 2 . 1 9

BGP packet header

Now that we have mentioned message types, we need to explain them.

BGP uses four different message types: open, update, notification, and
keepalives.

Open messages are sent to each other after the TCP connection is estab-

lished. The open messages must be accepted by the peer; otherwise, the
remaining messages, update, notification, and keepalives are not sent.

Update messages are the actual routing updates exchanged among BGP

peers. Other BGP-specific information is also included in these messages: ori-
gin, AS path, next hop, unreachable, and Inter-AS-metric.

Notification messages are used when errors are detected. Information

includes an explanation about why the connection was closed. Each reason
has a code. Notification updates use the following codes: message-header
error, open-message error, update-message error, and hold time expired.

Keepalive messages are sent to make peers aware that the connection can

be kept active. This is accomplished by updating the holddown timers.

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