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• Marker—contains
an authentication value that the message receiver can predict.
• Length—indicates
the total length of the message in bytes.
• Type—
specifies the message type as one of the following:
— Open
— Update
— Notification
— Keep-alive
• Data—Contains
upper-layer information in this optional field.
Open Message
Format
v
BGP
open messages are comprised of a BGP header and additional fields.
v The
below figure illustrates the additional fields used in BGP open messages.
BGP Open Message
Fields
BGP packets in
which the type field in the header identifies the packet to be a BGP open message
packet include the following fields. These fields provide the exchange criteria
for two BGP routers to establish a peer relationship.
• Version—Provides
the BGP version number so that the recipient can determine whether it is
running the same version as the sender.
• Autonomous
System—Provides the autonomous system number of the sender.
• Hold-Time—Indicates
the maximum number of seconds that can elapse without receipt of a message
before the transmitter is assumed to be nonfunctional.
• BGP
Identifier—Provides the BGP identifier of the sender (an IP
address), which is determined at startup and is identical for all local
interfaces and all BGP peers.
• Optional
Parameters Length—Indicates the length of the optional
parameters field (if present).
• Optional
Parameters—Contains a list of optional parameters (if any). Only
one optional parameter type is currently defined: authentication information.
Authentication
information consists of the following two fields:
— Authentication
code: Indicates the type of authentication being used.
Authentication
data: Contains data used by the authentication mechanism (if used).
Update Message
Format
v
BGP
update messages are comprised of a BGP header and additional fields.
v The
below figure illustrates the additional fields used in BGP update messages.
BGP Update
Message Fields
BGP packets in
which the type field in the header identifies the packet to be a BGP update message
packet include the following fields. Upon receiving an update message packet, routers
will be able to add or delete specific entries from their routing tables to
ensure accuracy. Update messages consist of the following fields:
• Unfeasible
Routes Length—Indicates the total length of the withdrawn routes field
or that the field is not present.
• Withdrawn
Routes—Contains a list of IP address prefixes for routes being
withdrawn from service.
• Total
Path Attribute Length—Indicates the total length of the path attributes
field or that the field is not present.
Path Attributes—Describes the
characteristics of the advertised path. The following are possible attributes
for a path:
— Origin:
Mandatory attribute that defines the origin of the path information
— AS Path:
Mandatory attribute composed of a sequence of autonomous system path segments
— Next Hop:
Mandatory attribute that defines the IP address of the border router that
should be used as the next hop to destinations listed in the network layer
reachability information field
— Mult Exit Disc:
Optional attribute used to discriminate between multiple exit points to a neighboring
autonomous system
— Local Pref:
Discretionary attribute used to specify the degree of preference for an advertised
route
— Atomic
Aggregate: Discretionary attribute used to disclose information about route selections
— Aggregator:
Optional attribute that contains information about aggregate routes
• Network
Layer Reachability Information—Contains a list of IP address prefixes
for the advertised routes
Notification
Message Format
The below figure illustrates the additional
fields used in BGP notification messages.
BGP Notification
Message Fields
BGP packets in
which the type field in the header identifies the packet to be a BGP
notification
message packet include the following fields. This packet is used to indicate
some sort of error condition to the peers of the originating router.
• Error
Code—Indicates the type of error that occurred. The following
are the error types defined by the field:
— Message Header
Error: Indicates a problem with a message header, such as unacceptable message
length, unacceptable marker field value, or unacceptable message type.
— Open Message
Error: Indicates a problem with an open message, such as unsupported version
number, unacceptable autonomous system number or IP address, or unsupported authentication
code.
— Update Message
Error: Indicates a problem with an update message, such as a malformed attribute
list, attribute list error, or invalid next-hop attribute.
— Hold Time
Expired: Indicates that the hold-time has expired, after which time a BGP node will
be considered nonfunctional.
— Finite State
Machine Error: Indicates an unexpected event.
— Cease: Closes a
BGP connection at the request of a BGP device in the absence of any fatal errors.
• Error
Subcode—Provides more specific information about the nature of
the reported error.
• Error
Data—Contains data based on the error code and error subcode
fields. This field is used to diagnose the reason for the notification message.
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