Editing Border Gateway Protocol (section)

== Packet format ==
=== Message header format ===

{| class=wikitable style="text-align:center; border:none; background:none; color: inherit;"
|+BGP version 4 message header format{{Ref RFC|4271|section=4.1|notes=no}}
|-
! style="width:4%;"|bit offset
! colspan="16" style="width:50%;"| 0–15
! colspan="8" style="width:25%;"| 16–23
! colspan="8" style="width:25%;"| 24–31
|-
! 0
| rowspan="4" colspan="32"| Marker (always: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff)
|-
! 32
|-
! 64
|-
! 96
|-
! 128
| colspan="16"| Length
| colspan="8"| Type
| colspan="8" style="border: none; background:none; color: inherit;"|
|}
* '''Marker''': Included for compatibility, must be set to all ones.
* '''Length''': Total length of the message in [[Octet (computing)|octets]], including the header.
* '''Type''': Type of BGP message. The following values are defined:
** Open (1)
** Update (2)
** Notification (3)
** KeepAlive (4)
** Route-Refresh (5)

note: "Marker" and "Length" is omitted from the examples.

=== Open Packet ===

; Version (8bit): Version of BGP used.
; My AS (16bit): Senders [[autonomous system (Internet)|autonomous system]] number.
; Hold Time (16bit): Timeout timer, used to calculate  KeepAlive messages. Default 90 seconds.
; BGP Identifier (32bit): IP-address of sender.
: Optional Parameters Length (8 bit): total length of the Optional parameters field.
'''Example of Open Message'''
 Type: Open Message (1)
 Version: 4
 My AS: 64496
 Hold Time: 90
 BGP Identifier: {{IPaddr|192.0.2.254}}
 Optional Parameters Length: 16
 Optional Parameters:
  Capability: Multiprotocol extensions capability (1)
  Capability: Route refresh capability (2)
  Capability: Route refresh capability (Cisco) (128)

=== Update Packet ===

Only changes are sent, after initial exchange, only difference (add/change/removed) are sent.

'''Example of UPDATE Message'''
 Type: UPDATE Message (2)
 Withdrawn Routes Length: 0
 Total Path Attribute Length: 25
 Path attributes
  ORIGIN: IGP
  AS_PATH: 64500
  NEXT_HOP: 192.0.2.254
  MULTI_EXIT_DISC: 0
 Network Layer Reachability Information (NLRI)
  {{IPaddr|192.0.2.0/27}}
  {{IPaddr|192.0.2.32/27}}
  {{IPaddr|192.0.2.64/27}}

=== Notification ===

If there is an error it is because one of the fields in the OPEN or UPDATE message does not match between the peers, e.g., BGP version mismatch, the peering router expects a different My AS, etc. The router then sends a Notification message to the peer indicating why the error occurred.

{| class="wikitable  mw-collapsible mw-collapsed" style="margin: 0 auto; text-align:center"
|+ Error Codes
! rowspan="2" |Error Code !! rowspan="2" |Name !! colspan="2" |subcodes
|-
|Code || Name
|-
|rowspan="3" | 1 || rowspan="3" |Message Header Error || 1 || style="text-align:left"| Connection Not Synchronized
|-
| 2 || style="text-align:left"| Bad Message Length
|-
| 3 || style="text-align:left"| Bad Message Type
|-
|rowspan="6" | 2 || rowspan="6" |OPEN Message Error || 1 || style="text-align:left"| Unsupported Version Number.
|-
|2 || style="text-align:left"| Bad Peer AS.
|-
| 3 || style="text-align:left"| Bad BGP Identifier.
|-
| 4 || style="text-align:left"|Unsupported Authentication Code.
|-
| 5 || style="text-align:left"|Authentication Failure.
|-
| 6 || style="text-align:left"|Unacceptable Hold Time.
|-
| rowspan="11" |3 || rowspan="11"; style="text-align:left"|UPDATE Message Error || 1 || style="text-align:left"|Malformed Attribute List.
|-
| 2 || style="text-align:left"|Unrecognized Well-known Attribute.
|-
| 3 || style="text-align:left"|Missing Well-known Attribute.
|-
| 4 || style="text-align:left"|Attribute Flags Error.
|-
| 5 || style="text-align:left"|Attribute Length Error.
|-
| 6 || style="text-align:left"|Invalid ORIGIN Attribute
|-
| 7 || style="text-align:left"|AS Routing Loop.
|-
| 8 || style="text-align:left"|Invalid NEXT_HOP Attribute.
|-
| 9 || style="text-align:left"|Optional Attribute Error.
|-
| 10 || style="text-align:left"|Invalid Network Field.
|-
| 11 || style="text-align:left"|Malformed AS_PATH.
|-
| 4 || colspan="3" |Hold Timer Expired
|-
| 5 || colspan="3" |Finite State Machine Error
|-
| 6 || colspan="3" |Cease
|}

'''Example of NOTIFICATION Message'''
 Type: NOTIFICATION Message (3)
 Major error Code: OPEN Message Error (2)
 Minor error Code (Open Message): Bad Peer AS (2)
 Bad Peer AS: 65200

=== KeepAlive ===
KeepAlive messages are sent periodically, to verify that remote peer is still alive.
keepalives should be sent at intervals of one third the <code>holdtime</code>.

'''Example of KEEPALIVE Message'''
 Type: KEEPALIVE Message (4)

=== Route-Refresh ===

Defined in RFC{{IETF RFC|7313|plainlink=yes}}.

Allows for soft updating of <code>Adj-RIB-in</code>, without resetting connection.

'''Example of ROUTE-REFRESH Message'''
 Type: ROUTE-REFRESH Message (5)
 Address family identifier (AFI): IPv4 (1)
 Subtype: Normal route refresh request [RFC2918] with/without ORF [RFC5291] (0)
 Subsequent address family identifier (SAFI): Unicast (1)