    Network Working Group                                        K. Lougheed
    Request for Comments:  1105                                cisco Systems
                                                                  Y. Rekhter
                                      T.J. Watson Research Center, IBM Corp.
                                                                   June 1989

                        A Border Gateway Protocol (BGP)

    Status of this Memo

       This RFC outlines a specific approach for the exchange of network
       reachability information between Autonomous Systems.

       At the time of this writing, the Border Gateway Protocol
       implementations exist for cisco routers as well as for the NSFNET
       Nodal Switching Systems.  A public domain version for "gated" is
       currently being implemented.

       Distribution of this memo is unlimited.

    1. Introduction

       The Border Gateway Protocol (BGP) is an inter-autonomous system
       routing protocol.  It is built on experience gained with EGP as
       defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
       described in RFC 1092 [2] and RFC 1093 [3].

       The primary function of a BGP speaking system is to exchange network
       reachability information with other BGP systems.  This network
       reachability information includes information on the autonomous
       systems (AS's) that traffic must transit to reach these networks.
       This information is sufficient to construct a graph of AS
       connectivity from which routing loops may be pruned and policy
       decisions at an AS level may be enforced.

       BGP runs over a reliable transport level protocol.  This eliminates
       the need to implement explicit update fragmentation, retransmission,
       acknowledgement, and sequencing.  Any authentication scheme used by
       the transport protocol may be used in addition to BGP's own
       authentication mechanisms.

       The initial BGP implementation is based on TCP [4], however any
       reliable transport may be used.  A message passing protocol such as
       VMTP [5] might be more natural for BGP.  TCP will be used, however,
       since it is present in virtually all commercial routers and hosts.
       In the following descriptions the phrase "transport protocol
       connection" can be understood to refer to a TCP connection.  BGP uses
       TCP port 179 for establishing its connections.

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    RFC 1105                          BGP                          June 1989

    2. Summary of Operation

       Two hosts form a transport protocol connection between one another.
       They exchange messages to open and confirm the connection parameters.
       The initial data flow is the entire BGP routing table.  Incremental
       updates are sent as the routing tables change.  Keepalive messages
       are sent periodically to ensure the liveness of the connection.
       Notification messages are sent in response to errors or special
       conditions.  If a connection encounters an error condition, a
       notification message is sent and the connection is optionally closed.

       The hosts executing the Border Gateway Protocol need not be routers.
       A non-routing host could exchange routing information with routers
       via EGP or even an interior routing protocol.  That non-routing host
       could then use BGP to exchange routing information with a border
       gateway in another autonomous system.  The implications and
       applications of this architecture are for further study.

       If a particular AS has more than one BGP gateway, then all these
       gateways should have a consistent view of routing.  A consistent view
       of the interior routes of the autonomous system is provided by the
       intra-AS routing protocol.  A consistent view of the routes exterior
       to the AS may be provided in a variety of ways.  One way is to use
       the BGP protocol to exchange routing information between the BGP
       gateways within a single AS.  In this case, in order to maintain
       consist routing information, these gateways MUST have direct BGP
       sessions with each other (the BGP sessions should form a complete
       graph).  Note that this requirement does not imply that all BGP
       gateways within a single AS must have direct links to each other;
       other methods may be used to ensure consistent routing information.

    3. Message Formats

       This section describes message formats and actions to be taken when
       errors are detected while processing these messages.

       Messages are sent over a reliable transport protocol connection.  A
       message is processed after it is entirely received.  The maximum
       message size is 1024 bytes.  All implementations are required to
       support this maximum message size.  The smallest message that may be
       sent consists of a BGP header without a data portion, or 8 bytes.

       The phrase "the BGP connection is closed" means that the transport
       protocol connection has been closed and that all resources for that
       BGP connection have been deallocated.  Routing table entries
       associated with the remote peer are marked as invalid.  This
       information is passed to other BGP peers before being deleted from
       the system.

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    RFC 1105                          BGP                          June 1989

    3.1 Message Header Format

       Each message has a fixed size header.  There may or may not be a data
       portion following the header, depending on the message type.  The
       layout of these fields is shown below.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        Marker                |          Length                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Version   |     Type      |        Hold Time               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Marker: 16 bits

          The Marker field is 16 bits of all ones.  This field is used to
          mark the start of a message.  If the first two bytes of a message
          are not all ones then we have a synchronization error and the BGP
          connection should be closed after sending a notification message
          with opcode 5 (connection not synchronized).  No notification data
          is sent.

       Length: 16 bits

          The Length field is 16 bits.  It is the total length of the
          message, incluluding header, in bytes.  If an illegal length is
          encountered (more than 1024 bytes or less than 8 bytes), a
          notification message with opcode 6 (bad message length) and two
          data bytes of the bad length should be sent and the BGP connection
          closed.

       Version: 8 bits

          The Version field is 8 bits of protocol version number.  The
          current BGP version number is 1.  If a bad version number is
          found, a notification message with opcode 8 (bad version number)
          should be sent and the BGP connection closed.  The bad version
          number should be included in one byte of notification data.

       Type: 8 bits

          The Type field is 8 bits of message type code.  The following type
          codes are defined:

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    RFC 1105                          BGP                          June 1989

                        1 - OPEN
                        2 - UPDATE
                        3 - NOTIFICATION
                        4 - KEEPALIVE
                        5 - OPEN CONFIRM

          If an unrecognized type value is found, a notification message
          with opcode 7 (bad type code) and data consisting of the byte of
          type field in question should be sent and the BGP connection
          closed.

       Hold Timer: 16 bits.

          This field contains the number of seconds that may elapse since
          receiving a BGP KEEPALIVE or BGP UPDATE message from our BGP peer
          before we declare an error and close the BGP connection.

    3.2  OPEN Message Format

       After a transport protocol connection is established, the first
       message sent by either side is an OPEN message.  If the OPEN message
       is acceptable, an OPEN CONFIRM message confirming the OPEN is sent
       back.  Once the OPEN is confirmed, UPDATE, KEEPALIVE, and
       NOTIFICATION messages may be exchanged.

       In addition to the fixed size BGP header, the OPEN message contains
       the following fields.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    My Autonomous System      |   Link Type   |  Auth. Code    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                 Authentication Data                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       My Autonomous System: 16 bits

          This field is our 16 bit autonomous system number.  If there is a
          problem with this field, a notification message with opcode 9
          (invalid AS field) should be sent and the BGP connection closed.
          No notification data is sent.

       Link Type: 8 bits

          The Link Type field is a single octet containing one of the

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    RFC 1105                          BGP                          June 1989

          following codes defining our position in the AS graph relative to
          our peer.

                           0  - INTERNAL
                           1  - UP
                           2  - DOWN
                           3  - H-LINK

          UP indicates the peer is higher in the AS hierarchy, DOWN
          indicates lower, and H-LINK indicates at the same level.  INTERNAL
          indicates that the peer is another BGP speaking host in our
          autonomous system.  INTERNAL links are used to keep AS routing
          information consistent with an AS with multiple border gateways.
          If the Link Type field is unacceptable, a notification message
          with opcode 1 (link type error in open) and data consisting of the
          expected link type should be sent and the BGP connection closed.
          The acceptable values for the Link Type fields of two BGP peers
          are discussed below.

       Authentication Code: 8 bits

          The Authentication Code field is an octet whose value describes
          the authentication mechanism being used.  A value of zero
          indicates no BGP authentication.  Note that a separate
          authentication mechanism may be used in establishing the transport
          level connection.  If the authentication code is not recognized, a
          notification message with opcode 2 (unknown authentication code)
          and no data is sent and the BGP connection is closed.

       Authentication Data: variable length

          The Authentication Data field is a variable length field
          containing authentication data.  If the value of Authentication
          Code field is zero, the Authentication Data field has zero length.
          If authentication fails, a notification message with opcode 3
          (authentication failure) and no data is sent and the BGP
          connection is closed.

    3.3 OPEN CONFIRM Message Format

       An OPEN CONFIRM message is sent after receiving an OPEN message.
       This completes the BGP connection setup.  UPDATE, NOTIFICATION, and
       KEEPALIVE messages may now be exchanged.

       An OPEN CONFIRM message consists of a BGP header with an OPEN CONFIRM
       type code.  There is no data in an OPEN CONFIRM message.

    Lougheed & Rekhter                                              [Page 5]

    RFC 1105                          BGP                          June 1989

    3.4 UPDATE Message Format

       UPDATE messages are used to transfer routing information between BGP
       peers.  The information in the UPDATE packet can be used to construct
       a graph describing the relationships of the various autonomous
       systems.  By applying rules to be discussed, routing information
       loops and some other anomalies may be detected and removed from the
       inter-AS routing.

       Whenever an error in a UPDATE message is detected, a notification
       message is sent with opcode 4 (bad update), a two byte subcode
       describing the nature of the problem, and a data field consisting of
       as much of the UPDATE message data portion as possible.  UPDATE
       messages have the following format:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Gateway                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   AS count    | Direction     |         AS Number             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     repeat (Direction, AS Number) pairs AS count times        |
       /                                                               /
       /                                                               /
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Net Count                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Network                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Metric                   |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |       repeat (Network, Metric) pairs Net Count times          |
       /                                                               /
       /                                                               /
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Gateway: 32 bits.

          The Gateway field is the address of a gateway that has routes to
          the Internet networks listed in the rest of the UPDATE message.
          This gateway MUST belong to the same AS as the BGP peer who
          advertises it.  If there is a problem with the gateway field, a
          notification message with subcode 6 (invalid gateway field) is
          sent.

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    RFC 1105                          BGP                          June 1989

       AS count: 8 bits.

          This field is the count of Direction and AS Number pairs in this
          UPDATE message.  If an incorrect AS count field is detected,
          subcode 1 (invalid AS count) is specified in the notification
          message.

       Direction: 8 bits

          The Direction field is an octet containing the direction taken by
          the routing information when exiting the AS defined by the
          succeeding AS Number field.  The following values are defined.

                1  - UP            (went up a link in the graph)
                2  - DOWN          (went down a link in the graph)
                3  - H_LINK        (horizontal link in the graph)
                4  - EGP_LINK      (EGP derived information)
                5  - INCOMPLETE    (incomplete information)

          There is a special provision to pass exterior learned (non-BGP)
          routes over BGP.  If an EGP learned route is passed over BGP, then
          the Direction field is set to EGP-LINK and the AS Number field is
          set to the AS number of the EGP peer that advertised this route.
          All other exterior-learned routes (non-BGP and non-EGP) may be
          passed by setting AS Number field to zero and Direction field to
          INCOMPLETE.  If the direction code is not recognized, a
          notification message with subcode 2 (invalid direction code) is
          sent.

       AS Number: 16 bits

          This field is the AS number that transmitted the routing
          information.  If there is a problem with this AS number, a
          notification message with subcode 3 (invalid autonomous system) is
          sent.

       Net Count: 16 bits.

          The Net Count field is the number of Metric and Network field
          pairs which follow this field.  If there is a problem with this
          field, a notification with subcode 7 (invalid net count field) is
          sent.

       Network: 32 bits

          The Network field is four bytes of Internet network number.  If
          there is a problem with the network field, a notification message
          with subcode 8 (invalid network field) is sent.

    Lougheed & Rekhter                                              [Page 7]

    RFC 1105                          BGP                          June 1989

       Metric: 16 bits

          The Metric field is 16 bits of an unspecified metric.  BGP metrics
          are comparable ONLY if routes have exactly the same AS path.  A
          metric of all ones indicates the network is unreachable.  In all
          other cases the metric field is MEANINGLESS and MUST BE IGNORED.
          There are no illegal metric values.

    3.5  NOTIFICATION Message Format

       NOTIFICATION messages are sent when an error condition is detected.
       The BGP connection is closed shortly after sending the notification
       message.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Opcode               |           Data                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Opcode: 16 bits

          The Opcode field describes the type of NOTIFICATION.  The
          following opcodes have been defined.

                1 (*) - link type error in open.  Data is one byte of proper
                        link type.
                2 (*) - unknown authentication code.  No data.
                3 (*) - authentication failure.  No data.
                4     - update error.  See below for data description.
                5 (*) - connection out of sync.  No data.
                6 (*) - invalid message length.  Data is two bytes of
                        bad length.
                7 (*) - invalid message type.  Data is one byte of bad
                        message type.
                8 (*) - invalid version number.  Data is one byte of
                        bad version.
                9 (*) - invalid AS field in OPEN.  No data.
               10 (*) - BGP Cease.  No data.

          The starred opcodes in the list above are considered fatal errors
          and cause transport connection termination.

          The update error (opcode 4) has as data 16 bits of subcode
          followed by the last UPDATE message in question.  After the
          subcode comes as much of the data portion of the UPDATE in

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    RFC 1105                          BGP                          June 1989

          question as possible.  The following subcodes are defined:

                   1 - invalid AS count
                   2 - invalid direction code
                   3 - invalid autonomous system
                   4 - EGP_LINK or INCOMPLETE_LINK link type at other than
                       the end of the AS path list
                   5 - routing loop
                   6 - invalid gateway field
                   7 - invalid Net Count field
                   8 - invalid network field

       Data: variable

          The Data field contains zero or more bytes of data to be used in
          diagnosing the reason for the NOTIFICATION.  The contents of the
          Data field depend upon the opcode.  See the opcode descriptions
          above for more details.

    3.6 KEEPALIVE Message Format

       BGP does not use any transport protocol based keepalive mechanism to
       determine if peers are reachable.  Instead KEEPALIVE messages are
       exchanged between peers often enough as not to cause the hold time
       (as advertised in the BGP header) to expire.  A reasonable minimum
       frequency of KEEPALIVE exchange would be one third of the Hold Time
       interval.

       As soon as the Hold Time associated with BGP peer has expired, the
       BGP connection is closed and BGP deallocates all resources associated
       with this peer.

       The KEEPALIVE message is a BGP header without any data.

    4. BGP Finite State machine.

       This section specifies BGP operation in terms of a Finite State
       Machine (FSM).  Following is a brief summary and overview of BGP
       operations by state as determined by this FSM.  A condensed version
       of the BGP FSM is found in Appendix 1.

       Initially BGP is in the BGP_Idle state.

       BGP_Idle state:

          In this state BGP refuses all incoming BGP connections.  No
          resources are allocated to the BGP neighbor.  In response to the
          Start event (initiated by either system or operator) the local

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    RFC 1105                          BGP                          June 1989

          system initializes all BGP resources and changes its state to
          BGP_Active.

       BGP_Active state:

          In this state BGP is trying to acquire a BGP neighbor by opening a
          transport protocol connection.  If the transport protocol open
          fails (for example, retransmission timeout),  BGP stays in the
          BGP_Active state.

          Otherwise,  the local system sends an OPEN message to its peer,
          and changes its state to BGP_OpenSent.  Since the hold time of the
          peer is still undetermined, the hold time is initialized to some
          large value.

          In response to the Stop event (initiated by either system or
          operator) the local system releases all BGP resources and changes
          its state to BGP_Idle.

       BGP_OpenSent state:

          In this state BGP waits for an OPEN message from its peer.  When
          an OPEN message is received, all fields are checked for
          correctness.  If the initial BGP header checking detects an error,
          BGP deallocates all resources associated with this peer and
          returns to the BGP_Active state.  Otherwise, the Link Type,
          Authentication Code, and Authentication Data fields are checked
          for correctness.

          If the link type is incorrect, a NOTIFICATION message with opcode
          1 (link type error in open) is sent.  The following combination of
          link type fields are correct; all other combinations are invalid.

                          Our view         Peer view
                          UP                DOWN
                          DOWN              UP
                          INTERNAL          INTERNAL
                          H-LINK            H-LINK

          If the link between two peers is INTERNAL, then AS number of both
          peers must be the same.  Otherwise, a NOTIFICATION message with
          opcode 1 (link type error in open) is sent.

          If both peers have the same AS number and the link type between
          these peers is not INTERNAL, then a NOTIFICATION message with
          opcode 1 (link type error in open) is sent.

          If the value of the Authentication Code field is zero, any

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    RFC 1105                          BGP                          June 1989

          information in the Authentication Data field (if present) is
          ignored.  If the Authentication Code field is non-zero it is
          checked for known authentication codes.  If authentication code is
          unknown, then the BGP NOTIFICATION message with opcode 2 (unknown
          authentication code) is sent.

          If the Authentication Code value is non-zero, then the
          corresponding authentication procedure is invoked.  The default
          values are a zero Authentication Code and no Authentication Data.

          If any of the above tests detect an error, the local system closes
          the BGP connection and changes its state to BGP_Idle.

          If there are no errors in the BGP OPEN message, BGP sends an OPEN
          CONFIRM message and goes into the BGP_OpenConfirm state.  At this
          point the hold timer which was originally set to some arbitrary
          large value (see above) is replaced with the value indicated in
          the OPEN message.

          If disconnect notification is received from the underlying
          transport protocol or if the hold time expires, the local system
          closes the BGP connection and changes its state to BGP_Idle.

       BGP_OpenConfirm state:

          In this state BGP waits for an OPEN CONFIRM message.  As soon as
          this message is received, BGP changes its state to
          BGP_Established.  If the hold timer expires before an OPEN CONFIRM
          message is received, the local system closes the BGP connection
          and changes its state to BGP_Idle.

       BGP_Established state:

          In the BGP_Established state BGP can exchange UPDATE,
          NOTIFICATION, and KEEPALIVE messages with its peer.

          If disconnect notification is received from the underlying
          transport protocol or if the hold time expires, the local system
          closes the BGP connection and changes its state to BGP_Idle.

          In response to the Stop event initiated by either the system or
          operator, the local system sends a NOTIFICATION message with
          opcode 10 (BGP Cease), closes the BGP connection, and changes its
          state to BGP_Idle.

    Lougheed & Rekhter                                             [Page 11]

    RFC 1105                          BGP                          June 1989

    5. UPDATE Message Handling

       A BGP UPDATE message may be received only in the BGP_Established
       state.  When a BGP UPDATE message is received, each field is checked
       for validity.  When a NOTIFICATION message is sent regarding an
       UPDATE, the opcode is always 4 (update error), the subcode depends on
       the type of error, and the rest of the data field is as much as
       possible of the data portion of the UPDATE that caused the error.

       If the Gateway field is incorrect, a BGP NOTIFICATION message is sent
       with subcode 6 (invalid gateway field).  All information in this
       UPDATE message is discarded.

       If the AS Count field is less than or equal to zero, a BGP
       NOTIFICATION is sent with subcode 1 (invalid AS count).  Otherwise,
       the complete AS path is extracted and checked as described below.

       If one of the Direction fields in the AS route list is not defined, a
       BGP NOTIFICATION message is with subcode 2 (invalid direction code).

       If one of the AS Number fields in the AS route list is incorrect, a
       BGP NOTIFICATION message is sent with subcode 3 (invalid autonomous
       system).

       If either a EGP_LINK or a INCOMPLETE_LINK link type occurs at other
       than the end of the AS path, a BGP NOTIFICATION message is sent with
       subcode 4 (EGP_LINK or INCOMPLETE_LINK link type at other than the
       end of the AS path list).

       If none of the above tests failed, the full AS route is checked for
       AS loops.

       AS loop detection is done by scanning the full AS route and checking
       that each AS in this route occurs only once.  If an AS loop is
       detected, a BGP NOTIFICATION message is sent with subcode 5 (routing
       loop).

       If any of the above errors are detected, no further processing is
       done.  Otherwise, the complete AS path is correct and the rest of the
       UPDATE message is processed.

       If the Net Count field is incorrect, a BGP NOTIFICATION message is
       sent with subcode 7 (invalid Net Count field).

       Each network and metric pair listed in the BGP UPDATE message is
       checked for a valid network number.  If the Network field is
       incorrect, a BGP Notification message is sent with subcode 8 (invalid
       network field).  No checking is done on the metric field.  It is up

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    RFC 1105                          BGP                          June 1989

       to a particular implementation to decide whether to continue
       processing or terminate it upon the first incorrect network.

       If the network, its complete AS path, and the gateway are correct,
       then the route is compared with other routes to the same network.  If
       the new route is better than the current one, then it is flooded to
       other BGP peers as follows:

        - If the BGP UPDATE was received over the INTERNAL link, it is not
          propagated over any other INTERNAL link.  This restriction is
          due to the fact that all BGP gateways within a single AS
          form a completely connected graph (see above).

        - Before sending a BGP UPDATE message over the non-INTERNAL links,
          check the AS path to insure that doing so would not cause a
          routing loop.  The BGP UPDATE message is then propagated (subject
          to the local policy restrictions) over any of the non-INTERNAL
          link of a routing loop would not result.

        - If the BGP UPDATE message is propagated over a non-INTERNAL link,
          then the current AS number and link type of the link over which
          it is going to be propagated is prepended to the full AS path
          and the AS count field is incremented by 1.  If the BGP UPDATE
          message is propagated over an INTERNAL link, then the full AS
          path passed unmodified and the AS count stays the same.  The
          Gateway field is replaced with the sender's own address.

    6. Acknowledgements

       We would like to express our thanks to Len Bosack (cisco Systems),
       Jeff Honig (Cornell University) and all members of the IWG task force
       for their contributions to this document.

    Lougheed & Rekhter                                             [Page 13]

    RFC 1105                          BGP                          June 1989

                                    Appendix 1

    BGP FSM State Transitions and Actions.

       This Appendix discusses the transitions between states in the BGP FSM
       in response to BGP events.  The following is the list of these states
       and events.

           BGP States:

                1 - BGP_Idle
                2 - BGP_Active
                3 - BGP_OpenSent
                4 - BGP_OpenConfirm
                5 - BGP_Established

           BGP Events:

                1 - BGP Start
                2 - BGP Transport connection open
                3 - BGP Transport connection closed
                4 - BGP Transport connection open failed
                5 - Receive OPEN message
                6 - Receive OPEN CONFIRM message
                7 - Receive KEEPALIVE message
                8 - Receive UPDATE messages
                9 - Receive NOTIFICATION message
               10 - Holdtime timer expired
               11 - KeepAlive timer expired
               12 - Receive CEASE message
               13 - BGP Stop

       The following table describes the state transitions of the BGP FSM
       and the actions triggered by these transitions.

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    RFC 1105                          BGP                          June 1989

       Event                Actions               Message Sent   Next State
       --------------------------------------------------------------------
       BGP_Idle (1)
         1            Initialize resources           none             2
       BGP_Active (2)
         2           Initialize resources            OPEN             3
         4                   none                    none             2
        13           Release resources               none             1

       BGP_OpenSent(3)
        3                    none                    none             1
        5            Process OPEN is OK            OPEN CONFIRM       4
                     Process OPEN Message failed   NOTIFICATION       1
       11            Restart KeepAlive timer       KEEPALIVE          3
       13            Release resources               none             1

       BGP_OpenConfirm (4)
        6            Complete initialization         none             5
        3                   none                     none             1
       10            Close transport connection      none             1
       11            Restart KeepAlive timer       KEEPALIVE          4
       13            Release resources               none             1

       BGP_Established (5)
        7            Process KEEPALIVE               none             5
        8            Process UPDATE is OK          UPDATE             5
                     Process UPDATE failed         NOTIFICATION       5
        9            Process NOTIFICATION            none             5
       10            Close transport connection      none             1
       11            Restart KeepAlive timer       KEEPALIVE          5
       12            Close transport connection    NOTIFICATION       1
       13            Release resources               none             1
       --------------------------------------------------------------------

       All other state-event combinations are considered fatal errors and
       cause the termination of the BGP transport connection (if necessary)
       and a transition to the BGP_Idle state.

    Lougheed & Rekhter                                             [Page 15]

    RFC 1105                          BGP                          June 1989

       The following is a condensed version of the above state transition
       table.

       Events|BGP_Idle BGP_Active BGP_OpenSent BGP_OpenConfirm BGP_Estab
             |  (1)   |    (2)   |     (3)    |      (4)      |      (5)
             |-------------------------------------------------------------
        1    |   2    |          |            |               |
             |        |          |            |               |
        2    |        |     3    |            |               |
             |        |          |            |               |
        3    |        |          |      1     |       1       |
             |        |          |            |               |
        4    |        |     2    |            |               |
             |        |          |            |               |
        5    |        |          |    4 or 1  |               |
             |        |          |            |               |
        6    |        |          |            |       5       |
             |        |          |            |               |
        7    |        |          |            |               |       5
             |        |          |            |               |
        8    |        |          |            |               |       5
             |        |          |            |               |
        9    |        |          |            |               |       5
             |        |          |            |               |
       10    |        |          |            |       1       |       1
             |        |          |            |               |
       11    |        |          |      3     |       4       |       5
             |        |          |            |               |
       12    |        |          |            |               |       1
             |        |          |            |               |
       13    |        |     1    |      1     |       1       |       1
             |        |          |            |               |
             --------------------------------------------------------------

    Lougheed & Rekhter                                             [Page 16]

    RFC 1105                          BGP                          June 1989

    References

      [1]  Mills, D., "Exterior Gateway Protocol Formal Specification", RFC
           904, BBN, April 1984.

      [2]  Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
           Backbone", RFC 1092, T. J. Watson Research Center, February 1989.

      [3]  Braun, H-W., "The NSFNET Routing Architecture", RFC 1093,
           MERIT/NSFNET Project, February 1989.

      [4]  Postel, J., "Transmission Control Protocol - DARPA Internet
           Program Protocol Specification", RFC 793, DARPA, September 1981.

      [5]  Cheriton, D., "VMTP: Versatile Message Transaction Protocol", RFC
           1045, Stanford University, February 1988.

    Authors' Addresses

       Kirk Lougheed
       cisco Systems, Inc.
       1360 Willow Road, Suite 201
       Menlo Park, CA 94025

       Phone: (415) 326-1941

       Email: LOUGHEED@MATHOM.CISCO.COM

       Jacob Rekhter
       T.J. Watson Research Center
       IBM Corporation
       P.O. Box 218
       Yorktown Heights, NY 10598

       Phone: (914) 945-3896

       Email: YAKOV@IBM.COM

    Lougheed & Rekhter                                             [Page 17]