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RFC 1583

 
 
 

OSPF Version 2

Part 3 of 9, p. 50 to 75
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8.  Protocol Packet Processing

    This section discusses the general processing of OSPF routing
    protocol packets.  It is very important that the router topological
    databases remain synchronized.  For this reason, routing protocol
    packets should get preferential treatment over ordinary data
    packets, both in sending and receiving.

    Routing protocol packets are sent along adjacencies only (with the



          +---+            +---+
          |RT1|------------|RT2|            o---------------o
          +---+    N1      +---+           RT1             RT2



                                                 RT7
                                                  o---------+
            +---+   +---+   +---+                /|\        |
            |RT7|   |RT3|   |RT4|               / | \       |
            +---+   +---+   +---+              /  |  \      |
              |       |       |               /   |   \     |
         +-----------------------+        RT5o RT6o    oRT4 |
                  |       |     N2            *   *   *     |
                +---+   +---+                  *  *  *      |
                |RT5|   |RT6|                   * * *       |
                +---+   +---+                    ***        |
                                                  o---------+
                                                 RT3


                  Figure 10: The graph of adjacencies

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    exception of Hello packets, which are used to discover the
    adjacencies).  This means that all routing protocol packets travel a
    single IP hop, except those sent over virtual links.

    All routing protocol packets begin with a standard header.  The
    sections below give the details on how to fill in and verify this
    standard header.  Then, for each packet type, the section is listed
    that gives more details on that particular packet type's processing.

    8.1.  Sending protocol packets

        When a router sends a routing protocol packet, it fills in the
        fields of the standard OSPF packet header as follows.  For more
        details on the header format consult Section A.3.1:


        Version #
            Set to 2, the version number of the protocol as documented
            in this specification.

        Packet type
            The type of OSPF packet, such as Link state Update or Hello
            Packet.

        Packet length
            The length of the entire OSPF packet in bytes, including the
            standard OSPF packet header.

        Router ID
            The identity of the router itself (who is originating the
            packet).

        Area ID
            The OSPF area that the packet is being sent into.

        Checksum
            The standard IP 16-bit one's complement checksum of the
            entire OSPF packet, excluding the 64-bit authentication
            field.  This checksum should be calculated before handing
            the packet to the appropriate authentication procedure.

        AuType and Authentication
            Each OSPF packet exchange is authenticated.  Authentication
            types are assigned by the protocol and documented in
            Appendix D.  A different authentication scheme can be used
            for each OSPF area.  The 64-bit authentication field is set
            by the appropriate authentication procedure (determined by
            AuType).  This procedure should be the last called when

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            forming the packet to be sent.  The setting of the
            authentication field is determined by the packet contents
            and the authentication key (which is configurable on a per-
            interface basis).


        The IP destination address for the packet is selected as
        follows.  On physical point-to-point networks, the IP
        destination is always set to the address AllSPFRouters.  On all
        other network types (including virtual links), the majority of
        OSPF packets are sent as unicasts, i.e., sent directly to the
        other end of the adjacency.  In this case, the IP destination is
        just the Neighbor IP address associated with the other end of
        the adjacency (see Section 10).  The only packets not sent as
        unicasts are on broadcast networks; on these networks Hello
        packets are sent to the multicast destination AllSPFRouters, the
        Designated Router and its Backup send both Link State Update
        Packets and Link State Acknowledgment Packets to the multicast
        address AllSPFRouters, while all other routers send both their
        Link State Update and Link State Acknowledgment Packets to the
        multicast address AllDRouters.

        Retransmissions of Link State Update packets are ALWAYS sent as
        unicasts.

        The IP source address should be set to the IP address of the
        sending interface.  Interfaces to unnumbered point-to-point
        networks have no associated IP address.  On these interfaces,
        the IP source should be set to any of the other IP addresses
        belonging to the router.  For this reason, there must be at
        least one IP address assigned to the router.[2] Note that, for
        most purposes, virtual links act precisely the same as
        unnumbered point-to-point networks.  However, each virtual link
        does have an IP interface address (discovered during the routing
        table build process) which is used as the IP source when sending
        packets over the virtual link.

        For more information on the format of specific OSPF packet
        types, consult the sections listed in Table 10.

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             Type   Packet name            detailed section (transmit)
             _________________________________________________________
             1      Hello                  Section  9.5
             2      Database description   Section 10.8
             3      Link state request     Section 10.9
             4      Link state update      Section 13.3
             5      Link state ack         Section 13.5


            Table 10: Sections describing OSPF protocol packet transmission.



    8.2.  Receiving protocol packets

        Whenever a protocol packet is received by the router it is
        marked with the interface it was received on.  For routers that
        have virtual links configured, it may not be immediately obvious
        which interface to associate the packet with.  For example,
        consider the Router RT11 depicted in Figure 6.  If RT11 receives
        an OSPF protocol packet on its interface to Network N8, it may
        want to associate the packet with the interface to Area 2, or
        with the virtual link to Router RT10 (which is part of the
        backbone).  In the following, we assume that the packet is
        initially associated with the non-virtual  link.[3]

        In order for the packet to be accepted at the IP level, it must
        pass a number of tests, even before the packet is passed to OSPF
        for processing:


        o   The IP checksum must be correct.

        o   The packet's IP destination address must be the IP address
            of the receiving interface, or one of the IP multicast
            addresses AllSPFRouters or AllDRouters.

        o   The IP protocol specified must be OSPF (89).

        o   Locally originated packets should not be passed on to OSPF.
            That is, the source IP address should be examined to make
            sure this is not a multicast packet that the router itself
            generated.


        Next, the OSPF packet header is verified.  The fields specified
        in the header must match those configured for the receiving

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        interface.  If they do not, the packet should be discarded:


        o   The version number field must specify protocol version 2.

        o   The 16-bit one's complement checksum of the OSPF packet's
            contents must be verified.  Remember that the 64-bit
            authentication field must be excluded from the checksum
            calculation.

        o   The Area ID found in the OSPF header must be verified.  If
            both of the following cases fail, the packet should be
            discarded.  The Area ID specified in the header must either:

            (1) Match the Area ID of the receiving interface.  In this
                case, the packet has been sent over a single hop.
                Therefore, the packet's IP source address must be on the
                same network as the receiving interface.  This can be
                determined by comparing the packet's IP source address
                to the interface's IP address, after masking both
                addresses with the interface mask.  This comparison
                should not be performed on point-to-point networks. On
                point-to-point networks, the interface addresses of each
                end of the link are assigned independently, if they are
                assigned at all.

            (2) Indicate the backbone.  In this case, the packet has
                been sent over a virtual link.  The receiving router
                must be an area border router, and the Router ID
                specified in the packet (the source router) must be the
                other end of a configured virtual link.  The receiving
                interface must also attach to the virtual link's
                configured Transit area.  If all of these checks
                succeed, the packet is accepted and is from now on
                associated with the virtual link (and the backbone
                area).

        o   Packets whose IP destination is AllDRouters should only be
            accepted if the state of the receiving interface is DR or
            Backup (see Section 9.1).

        o   The AuType specified in the packet must match the AuType
            specified for the associated area.


        Next, the packet must be authenticated.  This depends on the
        AuType specified (see Appendix D).  The authentication procedure
        may use an Authentication key, which can be configured on a

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        per-interface basis.  If the authentication fails, the packet
        should be discarded.

        If the packet type is Hello, it should then be further processed
        by the Hello Protocol (see Section 10.5).  All other packet
        types are sent/received only on adjacencies.  This means that
        the packet must have been sent by one of the router's active
        neighbors.  If the receiving interface is a multi-access network
        (either broadcast or non-broadcast) the sender is identified by
        the IP source address found in the packet's IP header.  If the
        receiving interface is a point-to-point link or a virtual link,
        the sender is identified by the Router ID (source router) found
        in the packet's OSPF header.  The data structure associated with
        the receiving interface contains the list of active neighbors.
        Packets not matching any active neighbor are discarded.

        At this point all received protocol packets are associated with
        an active neighbor.  For the further input processing of
        specific packet types, consult the sections listed in Table 11.



              Type   Packet name            detailed section (receive)
              ________________________________________________________
              1      Hello                  Section 10.5
              2      Database description   Section 10.6
              3      Link state request     Section 10.7
              4      Link state update      Section 13
              5      Link state ack         Section 13.7


            Table 11: Sections describing OSPF protocol packet reception.



9.  The Interface Data Structure

    An OSPF interface is the connection between a router and a network.
    There is a single OSPF interface structure for each attached
    network; each interface structure has at most one IP interface
    address (see below).  The support for multiple addresses on a single
    network is a matter for future consideration.

    An OSPF interface can be considered to belong to the area that
    contains the attached network.  All routing protocol packets
    originated by the router over this interface are labelled with the
    interface's Area ID.  One or more router adjacencies may develop
    over an interface.  A router's link state advertisements reflect the

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    state of its interfaces and their associated adjacencies.

    The following data items are associated with an interface.  Note
    that a number of these items are actually configuration for the
    attached network; those items must be the same for all routers
    connected to the network.


    Type
        The kind of network to which the interface attaches.  Its value
        is either broadcast, non-broadcast yet still multi-access,
        point-to-point or virtual link.

    State
        The functional level of an interface.  State determines whether
        or not full adjacencies are allowed to form over the interface.
        State is also reflected in the router's link state
        advertisements.

    IP interface address
        The IP address associated with the interface.  This appears as
        the IP source address in all routing protocol packets originated
        over this interface.  Interfaces to unnumbered point-to-point
        networks do not have an associated IP address.

    IP interface mask
        Also referred to as the subnet mask, this indicates the portion
        of the IP interface address that identifies the attached
        network.  Masking the IP interface address with the IP interface
        mask yields the IP network number of the attached network.  On
        point-to-point networks and virtual links, the IP interface mask
        is not defined. On these networks, the link itself is not
        assigned an IP network number, and so the addresses of each side
        of the link are assigned independently, if they are assigned at
        all.

    Area ID
        The Area ID of the area to which the attached network belongs.
        All routing protocol packets originating from the interface are
        labelled with this Area ID.

    HelloInterval
        The length of time, in seconds, between the Hello packets that
        the router sends on the interface.  Advertised in Hello packets
        sent out this interface.

    RouterDeadInterval
        The number of seconds before the router's neighbors will declare

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        it down, when they stop hearing the router's Hello Packets.
        Advertised in Hello packets sent out this interface.

    InfTransDelay
        The estimated number of seconds it takes to transmit a Link
        State Update Packet over this interface.  Link state
        advertisements contained in the Link State Update packet will
        have their age incremented by this amount before transmission.
        This value should take into account transmission and propagation
        delays; it must be greater than zero.

    Router Priority
        An 8-bit unsigned integer.  When two routers attached to a
        network both attempt to become Designated Router, the one with
        the highest Router Priority takes precedence.  A router whose
        Router Priority is set to 0 is ineligible to become Designated
        Router on the attached network.  Advertised in Hello packets
        sent out this interface.

    Hello Timer
        An interval timer that causes the interface to send a Hello
        packet.  This timer fires every HelloInterval seconds.  Note
        that on non-broadcast networks a separate Hello packet is sent
        to each qualified neighbor.

    Wait Timer
        A single shot timer that causes the interface to exit the
        Waiting state, and as a consequence select a Designated Router
        on the network.  The length of the timer is RouterDeadInterval
        seconds.

    List of neighboring routers
        The other routers attached to this network.  On multi-access
        networks, this list is formed by the Hello Protocol.
        Adjacencies will be formed to some of these neighbors.  The set
        of adjacent neighbors can be determined by an examination of all
        of the neighbors' states.

    Designated Router
        The Designated Router selected for the attached network.  The
        Designated Router is selected on all multi-access networks by
        the Hello Protocol.  Two pieces of identification are kept for
        the Designated Router: its Router ID and its IP interface
        address on the network.  The Designated Router advertises link
        state for the network; this network link state advertisement is
        labelled with the Designated Router's IP address.  The
        Designated Router is initialized to 0.0.0.0, which indicates the
        lack of a Designated Router.

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    Backup Designated Router
        The Backup Designated Router is also selected on all multi-
        access networks by the Hello Protocol.  All routers on the
        attached network become adjacent to both the Designated Router
        and the Backup Designated Router.  The Backup Designated Router
        becomes Designated Router when the current Designated Router
        fails.  The Backup Designated Router is initialized to 0.0.0.0,
        indicating the lack of a Backup Designated Router.

    Interface output cost(s)
        The cost of sending a data packet on the interface, expressed in
        the link state metric.  This is advertised as the link cost for
        this interface in the router links advertisement.  There may be
        a separate cost for each IP Type of Service.  The cost of an
        interface must be greater than zero.

    RxmtInterval
        The number of seconds between link state advertisement
        retransmissions, for adjacencies belonging to this interface.
        Also used when retransmitting Database Description and Link
        State Request Packets.

    Authentication key
        This configured data allows the authentication procedure to
        generate and/or verify the Authentication field in the OSPF
        header.  The Authentication key can be configured on a per-
        interface basis.  For example, if the AuType indicates simple
        password, the Authentication key would be a 64-bit password.
        This key would be inserted directly into the OSPF header when
        originating routing protocol packets, and there could be a
        separate password for each network.


    9.1.  Interface states

        The various states that router interfaces may attain is
        documented in this section.  The states are listed in order of
        progressing functionality.  For example, the inoperative state
        is listed first, followed by a list of intermediate states
        before the final, fully functional state is achieved.  The
        specification makes use of this ordering by sometimes making
        references such as "those interfaces in state greater than X".
        Figure 11 shows the graph of interface state changes.  The arcs
        of the graph are labelled with the event causing the state
        change.  These events are documented in Section 9.2.  The
        interface state machine is described in more detail in Section
        9.3.

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                                  +----+   UnloopInd   +--------+
                                  |Down|<--------------|Loopback|
                                  +----+               +--------+
                                     |
                                     |InterfaceUp
                          +-------+  |               +--------------+
                          |Waiting|<-+-------------->|Point-to-point|
                          +-------+                  +--------------+
                              |
                     WaitTimer|BackupSeen
                              |
                              |
                              |   NeighborChange
          +------+           +-+<---------------- +-------+
          |Backup|<----------|?|----------------->|DROther|
          +------+---------->+-+<-----+           +-------+
                    Neighbor  |       |
                    Change    |       |Neighbor
                              |       |Change
                              |     +--+
                              +---->|DR|
                                    +--+

                      Figure 11: Interface State changes

                 In addition to the state transitions pictured,
                 Event InterfaceDown always forces Down State, and
                 Event LoopInd always forces Loopback State


        Down
            This is the initial interface state.  In this state, the
            lower-level protocols have indicated that the interface is
            unusable.  No protocol traffic at all will be sent or
            received on such a interface.  In this state, interface
            parameters should be set to their initial values.  All
            interface timers should be disabled, and there should be no
            adjacencies associated with the interface.

        Loopback
            In this state, the router's interface to the network is
            looped back.  The interface may be looped back in hardware
            or software.  The interface will be unavailable for regular
            data traffic.  However, it may still be desirable to gain
            information on the quality of this interface, either through
            sending ICMP pings to the interface or through something
            like a bit error test.  For this reason, IP packets may

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            still be addressed to an interface in Loopback state.  To
            facilitate this, such interfaces are advertised in router
            links advertisements as single host routes, whose
            destination is the IP interface address.[4]

        Waiting
            In this state, the router is trying to determine the
            identity of the (Backup) Designated Router for the network.
            To do this, the router monitors the Hello Packets it
            receives.  The router is not allowed to elect a Backup
            Designated Router nor a Designated Router until it
            transitions out of Waiting state.  This prevents unnecessary
            changes of (Backup) Designated Router.

        Point-to-point
            In this state, the interface is operational, and connects
            either to a physical point-to-point network or to a virtual
            link.  Upon entering this state, the router attempts to form
            an adjacency with the neighboring router.  Hello Packets are
            sent to the neighbor every HelloInterval seconds.

        DR Other
            The interface is to a multi-access network on which another
            router has been selected to be the Designated Router.  In
            this state, the router itself has not been selected Backup
            Designated Router either.  The router forms adjacencies to
            both the Designated Router and the Backup Designated Router
            (if they exist).

        Backup
            In this state, the router itself is the Backup Designated
            Router on the attached network.  It will be promoted to
            Designated Router when the present Designated Router fails.
            The router establishes adjacencies to all other routers
            attached to the network.  The Backup Designated Router
            performs slightly different functions during the Flooding
            Procedure, as compared to the Designated Router (see Section
            13.3).  See Section 7.4 for more details on the functions
            performed by the Backup Designated Router.

        DR  In this state, this router itself is the Designated Router
            on the attached network.  Adjacencies are established to all
            other routers attached to the network.  The router must also
            originate a network links advertisement for the network
            node.  The advertisement will contain links to all routers
            (including the Designated Router itself) attached to the
            network.  See Section 7.3 for more details on the functions
            performed by the Designated Router.

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    9.2.  Events causing interface state changes

        State changes can be effected by a number of events.  These
        events are pictured as the labelled arcs in Figure 11.  The
        label definitions are listed below.  For a detailed explanation
        of the effect of these events on OSPF protocol operation,
        consult Section 9.3.


        InterfaceUp
            Lower-level protocols have indicated that the network
            interface is operational.  This enables the interface to
            transition out of Down state.  On virtual links, the
            interface operational indication is actually a result of the
            shortest path calculation (see Section 16.7).

        WaitTimer
            The Wait Timer has fired, indicating the end of the waiting
            period that is required before electing a (Backup)
            Designated Router.

        BackupSeen
            The router has detected the existence or non-existence of a
            Backup Designated Router for the network.  This is done in
            one of two ways.  First, an Hello Packet may be received
            from a neighbor claiming to be itself the Backup Designated
            Router.  Alternatively, an Hello Packet may be received from
            a neighbor claiming to be itself the Designated Router, and
            indicating that there is no Backup Designated Router.  In
            either case there must be bidirectional communication with
            the neighbor, i.e., the router must also appear in the
            neighbor's Hello Packet.  This event signals an end to the
            Waiting state.

        NeighborChange
            There has been a change in the set of bidirectional
            neighbors associated with the interface.  The (Backup)
            Designated Router needs to be recalculated.  The following
            neighbor changes lead to the NeighborChange event.  For an
            explanation of neighbor states, see Section 10.1.

            o   Bidirectional communication has been established to a
                neighbor.  In other words, the state of the neighbor has
                transitioned to 2-Way or higher.

            o   There is no longer bidirectional communication with a
                neighbor.  In other words, the state of the neighbor has
                transitioned to Init or lower.

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            o   One of the bidirectional neighbors is newly declaring
                itself as either Designated Router or Backup Designated
                Router.  This is detected through examination of that
                neighbor's Hello Packets.

            o   One of the bidirectional neighbors is no longer
                declaring itself as Designated Router, or is no longer
                declaring itself as Backup Designated Router.  This is
                again detected through examination of that neighbor's
                Hello Packets.

            o   The advertised Router Priority for a bidirectional
                neighbor has changed.  This is again detected through
                examination of that neighbor's Hello Packets.

        LoopInd
            An indication has been received that the interface is now
            looped back to itself.  This indication can be received
            either from network management or from the lower level
            protocols.

        UnloopInd
            An indication has been received that the interface is no
            longer looped back.  As with the LoopInd event, this
            indication can be received either from network management or
            from the lower level protocols.

        InterfaceDown
            Lower-level protocols indicate that this interface is no
            longer functional.  No matter what the current interface
            state is, the new interface state will be Down.


    9.3.  The Interface state machine

        A detailed description of the interface state changes follows.
        Each state change is invoked by an event (Section 9.2).  This
        event may produce different effects, depending on the current
        state of the interface.  For this reason, the state machine
        below is organized by current interface state and received
        event.  Each entry in the state machine describes the resulting
        new interface state and the required set of additional actions.

        When an interface's state changes, it may be necessary to
        originate a new router links advertisement.  See Section 12.4
        for more details.

        Some of the required actions below involve generating events for

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        the neighbor state machine.  For example, when an interface
        becomes inoperative, all neighbor connections associated with
        the interface must be destroyed.  For more information on the
        neighbor state machine, see Section 10.3.


         State(s):  Down

            Event:  InterfaceUp

        New state:  Depends upon action routine

           Action:  Start the interval Hello Timer, enabling the
                    periodic sending of Hello packets out the interface.
                    If the attached network is a physical point-to-point
                    network or virtual link, the interface state
                    transitions to Point-to-Point.  Else, if the router
                    is not eligible to become Designated Router the
                    interface state transitions to DR Other.

                    Otherwise, the attached network is multi-access and
                    the router is eligible to become Designated Router.
                    In this case, in an attempt to discover the attached
                    network's Designated Router the interface state is
                    set to Waiting and the single shot Wait Timer is
                    started.  If in addition the attached network is
                    non-broadcast, examine the configured list of
                    neighbors for this interface and generate the
                    neighbor event Start for each neighbor that is also
                    eligible to become Designated Router.


         State(s):  Waiting

            Event:  BackupSeen

        New state:  Depends upon action routine.

           Action:  Calculate the attached network's Backup Designated
                    Router and Designated Router, as shown in Section
                    9.4.  As a result of this calculation, the new state
                    of the interface will be either DR Other, Backup or
                    DR.


         State(s):  Waiting

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            Event:  WaitTimer

        New state:  Depends upon action routine.

           Action:  Calculate the attached network's Backup Designated
                    Router and Designated Router, as shown in Section
                    9.4.  As a result of this calculation, the new state
                    of the interface will be either DR Other, Backup or
                    DR.


         State(s):  DR Other, Backup or DR

            Event:  NeighborChange

        New state:  Depends upon action routine.

           Action:  Recalculate the attached network's Backup Designated
                    Router and Designated Router, as shown in Section
                    9.4.  As a result of this calculation, the new state
                    of the interface will be either DR Other, Backup or
                    DR.


         State(s):  Any State

            Event:  InterfaceDown

        New state:  Down

           Action:  All interface variables are reset, and interface
                    timers disabled.  Also, all neighbor connections
                    associated with the interface are destroyed.  This
                    is done by generating the event KillNbr on all
                    associated neighbors (see Section 10.2).


         State(s):  Any State

            Event:  LoopInd

        New state:  Loopback

           Action:  Since this interface is no longer connected to the
                    attached network the actions associated with the
                    above InterfaceDown event are executed.

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         State(s):  Loopback

            Event:  UnloopInd

        New state:  Down

           Action:  No actions are necessary.  For example, the
                    interface variables have already been reset upon
                    entering the Loopback state.  Note that reception of
                    an InterfaceUp event is necessary before the
                    interface again becomes fully functional.


    9.4.  Electing the Designated Router

        This section describes the algorithm used for calculating a
        network's Designated Router and Backup Designated Router.  This
        algorithm is invoked by the Interface state machine.  The
        initial time a router runs the election algorithm for a network,
        the network's Designated Router and Backup Designated Router are
        initialized to 0.0.0.0.  This indicates the lack of both a
        Designated Router and a Backup Designated Router.

        The Designated Router election algorithm proceeds as follows:
        Call the router doing the calculation Router X.  The list of
        neighbors attached to the network and having established
        bidirectional communication with Router X is examined.  This
        list is precisely the collection of Router X's neighbors (on
        this network) whose state is greater than or equal to 2-Way (see
        Section 10.1).  Router X itself is also considered to be on the
        list.  Discard all routers from the list that are ineligible to
        become Designated Router.  (Routers having Router Priority of 0
        are ineligible to become Designated Router.)  The following
        steps are then executed, considering only those routers that
        remain on the list:


        (1) Note the current values for the network's Designated Router
            and Backup Designated Router.  This is used later for
            comparison purposes.

        (2) Calculate the new Backup Designated Router for the network
            as follows.  Only those routers on the list that have not
            declared themselves to be Designated Router are eligible to
            become Backup Designated Router.  If one or more of these
            routers have declared themselves Backup Designated Router
            (i.e., they are currently listing themselves as Backup
            Designated Router, but not as Designated Router, in their

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            Hello Packets) the one having highest Router Priority is
            declared to be Backup Designated Router.  In case of a tie,
            the one having the highest Router ID is chosen.  If no
            routers have declared themselves Backup Designated Router,
            choose the router having highest Router Priority, (again
            excluding those routers who have declared themselves
            Designated Router), and again use the Router ID to break
            ties.

        (3) Calculate the new Designated Router for the network as
            follows.  If one or more of the routers have declared
            themselves Designated Router (i.e., they are currently
            listing themselves as Designated Router in their Hello
            Packets) the one having highest Router Priority is declared
            to be Designated Router.  In case of a tie, the one having
            the highest Router ID is chosen.  If no routers have
            declared themselves Designated Router, assign the Designated
            Router to be the same as the newly elected Backup Designated
            Router.

        (4) If Router X is now newly the Designated Router or newly the
            Backup Designated Router, or is now no longer the Designated
            Router or no longer the Backup Designated Router, repeat
            steps 2 and 3, and then proceed to step 5.  For example, if
            Router X is now the Designated Router, when step 2 is
            repeated X will no longer be eligible for Backup Designated
            Router election.  Among other things, this will ensure that
            no router will declare itself both Backup Designated Router
            and Designated Router.[5]

        (5) As a result of these calculations, the router itself may now
            be Designated Router or Backup Designated Router.  See
            Sections 7.3 and 7.4 for the additional duties this would
            entail.  The router's interface state should be set
            accordingly.  If the router itself is now Designated Router,
            the new interface state is DR.  If the router itself is now
            Backup Designated Router, the new interface state is Backup.
            Otherwise, the new interface state is DR Other.

        (6) If the attached network is non-broadcast, and the router
            itself has just become either Designated Router or Backup
            Designated Router, it must start sending Hello Packets to
            those neighbors that are not eligible to become Designated
            Router (see Section 9.5.1).  This is done by invoking the
            neighbor event Start for each neighbor having a Router
            Priority of 0.

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        (7) If the above calculations have caused the identity of either
            the Designated Router or Backup Designated Router to change,
            the set of adjacencies associated with this interface will
            need to be modified.  Some adjacencies may need to be
            formed, and others may need to be broken.  To accomplish
            this, invoke the event AdjOK?  on all neighbors whose state
            is at least 2-Way.  This will cause their eligibility for
            adjacency to be reexamined (see Sections 10.3 and 10.4).


        The reason behind the election algorithm's complexity is the
        desire for an orderly transition from Backup Designated Router
        to Designated Router, when the current Designated Router fails.
        This orderly transition is ensured through the introduction of
        hysteresis: no new Backup Designated Router can be chosen until
        the old Backup accepts its new Designated Router
        responsibilities.

        The above procedure may elect the same router to be both
        Designated Router and Backup Designated Router, although that
        router will never be the calculating router (Router X) itself.
        The elected Designated Router may not be the router having the
        highest Router Priority, nor will the Backup Designated Router
        necessarily have the second highest Router Priority.  If Router
        X is not itself eligible to become Designated Router, it is
        possible that neither a Backup Designated Router nor a
        Designated Router will be selected in the above procedure.  Note
        also that if Router X is the only attached router that is
        eligible to become Designated Router, it will select itself as
        Designated Router and there will be no Backup Designated Router
        for the network.


    9.5.  Sending Hello packets

        Hello packets are sent out each functioning router interface.
        They are used to discover and maintain neighbor
        relationships.[6] On multi-access networks, Hello Packets are
        also used to elect the Designated Router and Backup Designated
        Router, and in that way determine what adjacencies should be
        formed.

        The format of an Hello packet is detailed in Section A.3.2.  The
        Hello Packet contains the router's Router Priority (used in
        choosing the Designated Router), and the interval between Hello
        Packets sent out the interface (HelloInterval).  The Hello
        Packet also indicates how often a neighbor must be heard from to
        remain active (RouterDeadInterval).  Both HelloInterval and

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        RouterDeadInterval must be the same for all routers attached to
        a common network.  The Hello packet also contains the IP address
        mask of the attached network (Network Mask).  On unnumbered
        point-to-point networks and on virtual links this field should
        be set to 0.0.0.0.

        The Hello packet's Options field describes the router's optional
        OSPF capabilities.  There are currently two optional
        capabilities defined (see Sections 4.5 and A.2).  The T-bit of
        the Options field should be set if the router is capable of
        calculating separate routes for each IP TOS.  The E-bit should
        be set if and only if the attached area is capable of processing
        AS external advertisements (i.e., it is not a stub area).  If
        the E-bit is set incorrectly the neighboring routers will refuse
        to accept the Hello Packet (see Section 10.5).  The rest of the
        Hello Packet's Options field should be set to zero.

        In order to ensure two-way communication between adjacent
        routers, the Hello packet contains the list of all routers from
        which Hello Packets have been seen recently.  The Hello packet
        also contains the router's current choice for Designated Router
        and Backup Designated Router.  A value of 0.0.0.0 in these
        fields means that one has not yet been selected.

        On broadcast networks and physical point-to-point networks,
        Hello packets are sent every HelloInterval seconds to the IP
        multicast address AllSPFRouters.  On virtual links, Hello
        packets are sent as unicasts (addressed directly to the other
        end of the virtual link) every HelloInterval seconds.  On non-
        broadcast networks, the sending of Hello packets is more
        complicated.  This will be covered in the next section.


        9.5.1.  Sending Hello packets on non-broadcast networks

            Static configuration information is necessary in order for
            the Hello Protocol to function on non-broadcast networks
            (see Section C.5).  Every attached router which is eligible
            to become Designated Router has a configured list of all of
            its neighbors on the network.  Each listed neighbor is
            labelled with its Designated Router eligibility.

            The interface state must be at least Waiting for any Hello
            Packets to be sent.  Hello Packets are then sent directly
            (as unicasts) to some subset of a router's neighbors.
            Sometimes an Hello Packet is sent periodically on a timer;
            at other times it is sent as a response to a received Hello
            Packet.  A router's hello-sending behavior varies depending

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            on whether the router itself is eligible to become
            Designated Router.

            If the router is eligible to become Designated Router, it
            must periodically send Hello Packets to all neighbors that
            are also eligible.  In addition, if the router is itself the
            Designated Router or Backup Designated Router, it must also
            send periodic Hello Packets to all other neighbors.  This
            means that any two eligible routers are always exchanging
            Hello Packets, which is necessary for the correct operation
            of the Designated Router election algorithm.  To minimize
            the number of Hello Packets sent, the number of eligible
            routers on a non-broadcast network should be kept small.

            If the router is not eligible to become Designated Router,
            it must periodically send Hello Packets to both the
            Designated Router and the Backup Designated Router (if they
            exist).  It must also send an Hello Packet in reply to an
            Hello Packet received from any eligible neighbor (other than
            the current Designated Router and Backup Designated Router).
            This is needed to establish an initial bidirectional
            relationship with any potential Designated Router.

            When sending Hello packets periodically to any neighbor, the
            interval between Hello Packets is determined by the
            neighbor's state.  If the neighbor is in state Down, Hello
            Packets are sent every PollInterval seconds.  Otherwise,
            Hello Packets are sent every HelloInterval seconds.


10.  The Neighbor Data Structure

    An OSPF router converses with its neighboring routers.  Each
    separate conversation is described by a "neighbor data structure".
    Each conversation is bound to a particular OSPF router interface,
    and is identified either by the neighboring router's OSPF Router ID
    or by its Neighbor IP address (see below).  Thus if the OSPF router
    and another router have multiple attached networks in common,
    multiple conversations ensue, each described by a unique neighbor
    data structure.  Each separate conversation is loosely referred to
    in the text as being a separate "neighbor".

    The neighbor data structure contains all information pertinent to
    the forming or formed adjacency between the two neighbors.
    (However, remember that not all neighbors become adjacent.)  An
    adjacency can be viewed as a highly developed conversation between
    two routers.

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    State
        The functional level of the neighbor conversation.  This is
        described in more detail in Section 10.1.

    Inactivity Timer
        A single shot timer whose firing indicates that no Hello Packet
        has been seen from this neighbor recently.  The length of the
        timer is RouterDeadInterval seconds.

    Master/Slave
        When the two neighbors are exchanging databases, they form a
        master/slave relationship.  The master sends the first Database
        Description Packet, and is the only part that is allowed to
        retransmit.  The slave can only respond to the master's Database
        Description Packets.  The master/slave relationship is
        negotiated in state ExStart.

    DD Sequence Number
        A 32-bit number identifying individual Database Description
        packets.  When the neighbor state ExStart is entered, the DD
        sequence number should be set to a value not previously seen by
        the neighboring router.  One possible scheme is to use the
        machine's time of day counter.  The DD sequence number is then
        incremented by the master with each new Database Description
        packet sent.  The slave's DD sequence number indicates the last
        packet received from the master.  Only one packet is allowed
        outstanding at a time.

    Neighbor ID
        The OSPF Router ID of the neighboring router.  The Neighbor ID
        is learned when Hello packets are received from the neighbor, or
        is configured if this is a virtual adjacency (see Section C.4).

    Neighbor Priority
        The Router Priority of the neighboring router.  Contained in the
        neighbor's Hello packets, this item is used when selecting the
        Designated Router for the attached network.

    Neighbor IP address
        The IP address of the neighboring router's interface to the
        attached network.  Used as the Destination IP address when
        protocol packets are sent as unicasts along this adjacency.
        Also used in router links advertisements as the Link ID for the
        attached network if the neighboring router is selected to be
        Designated Router (see Section 12.4.1).  The Neighbor IP address
        is learned when Hello packets are received from the neighbor.
        For virtual links, the Neighbor IP address is learned during the
        routing table build process (see Section 15).

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    Neighbor Options
        The optional OSPF capabilities supported by the neighbor.
        Learned during the Database Exchange process (see Section 10.6).
        The neighbor's optional OSPF capabilities are also listed in its
        Hello packets.  This enables received Hello Packets to be
        rejected (i.e., neighbor relationships will not even start to
        form) if there is a mismatch in certain crucial OSPF
        capabilities (see Section 10.5).  The optional OSPF capabilities
        are documented in Section 4.5.

    Neighbor's Designated Router
        The neighbor's idea of the Designated Router.  If this is the
        neighbor itself, this is important in the local calculation of
        the Designated Router.  Defined only on multi-access networks.

    Neighbor's Backup Designated Router
        The neighbor's idea of the Backup Designated Router.  If this is
        the neighbor itself, this is important in the local calculation
        of the Backup Designated Router.  Defined only on multi-access
        networks.


    The next set of variables are lists of link state advertisements.
    These lists describe subsets of the area topological database.
    There can be five distinct types of link state advertisements in an
    area topological database: router links, network links, and Type 3
    and 4 summary links (all stored in the area data structure), and AS
    external links (stored in the global data structure).


    Link state retransmission list
        The list of link state advertisements that have been flooded but
        not acknowledged on this adjacency.  These will be retransmitted
        at intervals until they are acknowledged, or until the adjacency
        is destroyed.

    Database summary list
        The complete list of link state advertisements that make up the
        area topological database, at the moment the neighbor goes into
        Database Exchange state.  This list is sent to the neighbor in
        Database Description packets.

    Link state request list
        The list of link state advertisements that need to be received
        from this neighbor in order to synchronize the two neighbors'
        topological databases.  This list is created as Database
        Description packets are received, and is then sent to the
        neighbor in Link State Request packets.  The list is depleted as

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        appropriate Link State Update packets are received.


    10.1.  Neighbor states

        The state of a neighbor (really, the state of a conversation
        being held with a neighboring router) is documented in the
        following sections.  The states are listed in order of
        progressing functionality.  For example, the inoperative state
        is listed first, followed by a list of intermediate states
        before the final, fully functional state is achieved.  The
        specification makes use of this ordering by sometimes making
        references such as "those neighbors/adjacencies in state greater
        than X".  Figures 12 and 13 show the graph of neighbor state
        changes.  The arcs of the graphs are labelled with the event
        causing the state change.  The neighbor events are documented in
        Section 10.2.

        The graph in Figure 12 shows the state changes effected by the
        Hello Protocol.  The Hello Protocol is responsible for neighbor

                                   +----+
                                   |Down|
                                   +----+
                                     |                               | Start
                                     |        +-------+
                             Hello   |   +---->|Attempt|
                            Received |         +-------+
                                     |             |
                             +----+<-+             |HelloReceived
                             |Init|<---------------+
                             +----+<--------+
                                |           |
                                |2-Way      |1-Way
                                |Received   |Received
                                |           |
              +-------+         |        +-----+
              |ExStart|<--------+------->|2-Way|
              +-------+                  +-----+

              Figure 12: Neighbor state changes (Hello Protocol)

                  In addition to the state transitions pictured,
                  Event KillNbr always forces Down State,
                  Event InactivityTimer always forces Down State,
                  Event LLDown always forces Down State

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        acquisition and maintenance, and for ensuring two way
        communication between neighbors.

        The graph in Figure 13 shows the forming of an adjacency.  Not
        every two neighboring routers become adjacent (see Section
        10.4).  The adjacency starts to form when the neighbor is in
        state ExStart.  After the two routers discover their
        master/slave status, the state transitions to Exchange.  At this
        point the neighbor starts to be used in the flooding procedure,
        and the two neighboring routers begin synchronizing their
        databases.  When this synchronization is finished, the neighbor
        is in state Full and we say that the two routers are fully
        adjacent.  At this point the adjacency is listed in link state
        advertisements.

        For a more detailed description of neighbor state changes,
        together with the additional actions involved in each change,
        see Section 10.3.

                                  +-------+
                                  |ExStart|
                                  +-------+
                                    |
                     NegotiationDone|
                                    +->+--------+
                                       |Exchange|
                                    +--+--------+
                                    |
                            Exchange|
                              Done  |
                    +----+          |      +-------+
                    |Full|<---------+----->|Loading|
                    +----+<-+              +-------+
                            |  LoadingDone     |
                            +------------------+

            Figure 13: Neighbor state changes (Database Exchange)

                In addition to the state transitions pictured,
                Event SeqNumberMismatch forces ExStart state,
                Event BadLSReq forces ExStart state,
                Event 1-Way forces Init state,
                Event KillNbr always forces Down State,
                Event InactivityTimer always forces Down State,
                Event LLDown always forces Down State,
                Event AdjOK? leads to adjacency forming/breaking

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        Down
            This is the initial state of a neighbor conversation.  It
            indicates that there has been no recent information received
            from the neighbor.  On non-broadcast networks, Hello packets
            may still be sent to "Down" neighbors, although at a reduced
            frequency (see Section 9.5.1).

        Attempt
            This state is only valid for neighbors attached to non-
            broadcast networks.  It indicates that no recent information
            has been received from the neighbor, but that a more
            concerted effort should be made to contact the neighbor.
            This is done by sending the neighbor Hello packets at
            intervals of HelloInterval (see Section 9.5.1).

        Init
            In this state, an Hello packet has recently been seen from
            the neighbor.  However, bidirectional communication has not
            yet been established with the neighbor (i.e., the router
            itself did not appear in the neighbor's Hello packet).  All
            neighbors in this state (or higher) are listed in the Hello
            packets sent from the associated interface.

        2-Way
            In this state, communication between the two routers is
            bidirectional.  This has been assured by the operation of
            the Hello Protocol.  This is the most advanced state short
            of beginning adjacency establishment.  The (Backup)
            Designated Router is selected from the set of neighbors in
            state 2-Way or greater.

        ExStart
            This is the first step in creating an adjacency between the
            two neighboring routers.  The goal of this step is to decide
            which router is the master, and to decide upon the initial
            DD sequence number.  Neighbor conversations in this state or
            greater are called adjacencies.

        Exchange
            In this state the router is describing its entire link state
            database by sending Database Description packets to the
            neighbor.  Each Database Description Packet has a DD
            sequence number, and is explicitly acknowledged.  Only one
            Database Description Packet is allowed outstanding at any
            one time.  In this state, Link State Request Packets may
            also be sent asking for the neighbor's more recent
            advertisements.  All adjacencies in Exchange state or
            greater are used by the flooding procedure.  In fact, these

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            adjacencies are fully capable of transmitting and receiving
            all types of OSPF routing protocol packets.

        Loading
            In this state, Link State Request packets are sent to the
            neighbor asking for the more recent advertisements that have
            been discovered (but not yet received) in the Exchange
            state.

        Full
            In this state, the neighboring routers are fully adjacent.
            These adjacencies will now appear in router links and
            network links advertisements.




(page 75 continued on part 4)

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