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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: 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 below:
- Open Message - This message is used to
establish connections with peers and includes fields for the BGP
version number; 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 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
,
which 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: network-layer reachability information (NLRI),
path attributes, and withdrawn routes. These three elements are
described briefly in the following sections.
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