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BGP Connection Establishment: Open Messages
(Page 2 of 2)
BGP Open Message Format
The specific format for BGP Open
messages can be found in Table 137
and Figure 193.
Figure 193: BGP Open Message Format
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Table 137: BGP Open Message Format
Field
Name
|
Size (bytes)
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Description
|
Marker
|
16
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Marker: This large
field at the start of each BGP message is used for synchronization and
authentication. See
the discussion of the BGP general message format for details.
|
Length
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2
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Length:
The total length of the message in bytes, including the fields of the
header. Open messages are variable in length.
|
Type
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1
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Type: BGP message
type, value is 1 for Open messages.
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Version
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1
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Version:
Indicates the BGP version the sender of the Open message is using.
This field allows devices to reject connections with devices using versions
they may not be capable of understanding. The current value is 4, for
BGP-4, and is used by most, if not all, current BGP implementations.
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My Autonomous
System
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2
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My Autonomous System:
Identifies the autonomous system number of the sender of the Open
message. AS numbers are centrally
managed across the Internet in a manner
similar to how IP addresses are administered.
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Hold
Time
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2
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Hold
Time: The number of seconds that this device proposes to use
for the BGP hold timer, which specifies how long a BGP peer will allow
the connection to be left silent between receipt of BGP messages. A
BGP device may refuse a connection if it doesn't like the value that
its peer is suggesting; usually, however, the two devices agree to use
the smaller of the values suggested by each device.
The value must be at least 3 seconds, or 0. If 0, this specifies that
the hold timer is not used. See
the Keepalive message discussion for more on how the hold timer is used.
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BGP Identifier
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4
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BGP Identifier:
Identifies the specific BGP speaker. Recall that IP addresses are associated
with interfaces, not devices, so each router will have at least two
IP addresses. Normally the BGP identifier is chosen as one of these
addresses. Once chosen, this identifier is used for all BGP communications
with BGP peers. This includes BGP peers on the interface from which
the identifier was chosen, and also BGP peers on other interfaces as
well. So, if a BGP speaker with two interfaces has addresses IP1
and IP2, it will choose one as its identifier and use it on both
of its interfaces.
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Opt
Parm Len
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1
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Optional
Parameters Length: The number of bytes used for Optional
Parameters (see below). If 0, no optional parameters are in this
message.
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Optional
Parameters
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Variable
|
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BGP Open messages
currently use only one optional parameter: Authentication Information.
Its Parameter Value subfield contains a one-byte Authentication
Code sub-subfield, which specifies the type of authentication a
device wishes to use. Following this is a variable-length Authentication
Data sub-subfield. The Authentication Code specifies how
authentication is to be performed, including the meaning of the Authentication
Data field, and the manner in which Marker fields are to
be calculated.
Key Concept: BGP sessions begin with each peer in a connection sending the other a BGP Open message. The purpose of this message is to establish contact between devices, identify the sender of the message and its autonomous system, and negotiate important parameters that dictate how the session will be conducted. |
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Version 3.0 - Version Date: September 20, 2005
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