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Different message types play an essential role in BGP operation. Each
message type includes the BGP message header.
The message header contains only three fields. They are a 16-byte
Marker field, a 2-byte Length field, and a 1-byte Type field. The Marker
field is used either to authenticate incoming BGP messages or to
detect loss of synchronization between two BGP peers.
The Length field indicates the total BGP message length, including
the header. The smallest BGP message is 19 bytes, 16 + 2 + 1, and the
largest possible message is 4096 bytes.
The Type field can have four values, 1 to 4. Each of these values
corresponds to one of the four BGP message types, described as
follows:
- Open Message – This message is used to establish
connections with peers and includes fields for the BGP version
number, the AS number, hold time, and Router ID.
- Keepalive Message
– This message type is sent periodically
between peers to maintain connections and verify paths held by the
router sending the keepalive.
If the
periodic timer is set to a value of zero (0), no keepalives are
sent. The recommended keepalive interval is one third of the hold
time interval. The keepalive message is a 19-byte BGP message header
with no data following it.
- Notification Message
– This message type is used to inform
the receiving router of errors. This message includes a field for
error codes that can be used to troubleshoot BGP connections.

- Update Message
– The update messages contain all the
information BGP uses to construct a loop free picture of the
internetwork. There are three basic components of an update message.
They are network-layer reachability information (NLRI), path
attributes, and withdrawn routes. These three elements are described briefly in the
following sections.
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Interactive Media Activity
Drag and Drop: BGP Message Type
Upon completion of this activity, the student will be able to
identify the properties of BGP messages types.

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