| The BGP message header format is a
16-byte marker field, followed by a 2-byte length field and a 1-byte
type field. The figure illustrates the basic format of the BGP
message header.
There may or may not be a data
portion following the header, depending on the message type.
KEEPALIVE messages, for example, consist of the message header only,
with no following data.
The marker field is used to either
authenticate incoming BGP messages or to detect loss of
synchronization between two BGP peers. The marker field can have two
formats:
- If the type of the message is OPEN
or if the OPEN message has no authentication information, the
marker field must be all ones.
- Otherwise, the marker field will
be computed based on part of the authentication mechanism used.
The length indicates the total BGP
message length, including the header. The smallest BGP message is no
less than 19 bytes (16 + 2 + 1) and no greater than 4096.
The type indicates the message type,
from the following possibilities:
- OPEN
- UPDATE
- NOTIFICATION
- KEEPALIVE
The following sections examine the
purpose and format of each of the four message types in more detail.
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