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RFC 1583

 
 
 

OSPF Version 2

Part 8 of 9, p. 177 to 197
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A.3.4 The Link State Request packet

    Link State Request packets are OSPF packet type 3.  After exchanging
    Database Description packets with a neighboring router, a router may
    find that parts of its topological database are out of date.  The
    Link State Request packet is used to request the pieces of the
    neighbor's database that are more up to date.  Multiple Link State
    Request packets may need to be used.  The sending of Link State
    Request packets is the last step in bringing up an adjacency.

    A router that sends a Link State Request packet has in mind the
    precise instance of the database pieces it is requesting, defined by
    LS sequence number, LS checksum, and LS age, although these fields
    are not specified in the Link State Request Packet itself.  The
    router may receive even more recent instances in response.

    The sending of Link State Request packets is documented in Section
    10.9.  The reception of Link State Request packets is documented in
    Section 10.7.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       3       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          LS type                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Link State ID                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |


    Each advertisement requested is specified by its LS type, Link State
    ID, and Advertising Router.  This uniquely identifies the
    advertisement, but not its instance.  Link State Request packets are

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    understood to be requests for the most recent instance (whatever
    that might be).

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A.3.5 The Link State Update packet

    Link State Update packets are OSPF packet type 4.  These packets
    implement the flooding of link state advertisements.  Each Link
    State Update packet carries a collection of link state
    advertisements one hop further from its origin.  Several link state
    advertisements may be included in a single packet.

    Link State Update packets are multicast on those physical networks
    that support multicast/broadcast.  In order to make the flooding
    procedure reliable, flooded advertisements are acknowledged in Link
    State Acknowledgment packets.  If retransmission of certain
    advertisements is necessary, the retransmitted advertisements are
    always carried by unicast Link State Update packets.  For more
    information on the reliable flooding of link state advertisements,
    consult Section 13.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       4       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      # advertisements                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                                                            +-+
       |                  Link state advertisements                    |
       +-                                                            +-+
       |                              ...                              |



    # advertisements
        The number of link state advertisements included in this update.

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    The body of the Link State Update packet consists of a list of link
    state advertisements.  Each advertisement begins with a common 20
    byte header, the link state advertisement header.  This header is
    described in Section A.4.1.  Otherwise, the format of each of the
    five types of link state advertisements is different.  Their formats
    are described in Section A.4.

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A.3.6 The Link State Acknowledgment packet

    Link State Acknowledgment Packets are OSPF packet type 5.  To make
    the flooding of link state advertisements reliable, flooded
    advertisements are explicitly acknowledged.  This acknowledgment is
    accomplished through the sending and receiving of Link State
    Acknowledgment packets.  Multiple link state advertisements can be
    acknowledged in a single Link State Acknowledgment packet.

    Depending on the state of the sending interface and the source of
    the advertisements being acknowledged, a Link State Acknowledgment
    packet is sent either to the multicast address AllSPFRouters, to the
    multicast address AllDRouters, or as a unicast.  The sending of Link
    State Acknowledgement packets is documented in Section 13.5.  The
    reception of Link State Acknowledgement packets is documented in
    Section 13.7.

    The format of this packet is similar to that of the Data Description
    packet.  The body of both packets is simply a list of link state
    advertisement headers.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Version #   |       5       |         Packet length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Router ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Area ID                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Checksum            |             AuType            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Authentication                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                                                             -+
       |                             A                                 |
       +-                 Link State Advertisement                    -+
       |                           Header                              |
       +-                                                             -+
       |                                                               |
       +-                                                             -+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |

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    Each acknowledged link state advertisement is described by its link
    state advertisement header.  The link state advertisement header is
    documented in Section A.4.1.  It contains all the information
    required to uniquely identify both the advertisement and the
    advertisement's current instance.

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A.4 Link state advertisement formats

    There are five distinct types of link state advertisements.  Each
    link state advertisement begins with a standard 20-byte link state
    advertisement header.  This header is explained in Section A.4.1.
    Succeeding sections then diagram the separate link state
    advertisement types.

    Each link state advertisement describes a piece of the OSPF routing
    domain.  Every router originates a router links advertisement.  In
    addition, whenever the router is elected Designated Router, it
    originates a network links advertisement.  Other types of link state
    advertisements may also be originated (see Section 12.4).  All link
    state advertisements are then flooded throughout the OSPF routing
    domain.  The flooding algorithm is reliable, ensuring that all
    routers have the same collection of link state advertisements.  (See
    Section 13 for more information concerning the flooding algorithm).
    This collection of advertisements is called the link state (or
    topological) database.

    From the link state database, each router constructs a shortest path
    tree with itself as root.  This yields a routing table (see Section
    11).  For the details of the routing table build process, see
    Section 16.

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A.4.1 The Link State Advertisement header

    All link state advertisements begin with a common 20 byte header.
    This header contains enough information to uniquely identify the
    advertisement (LS type, Link State ID, and Advertising Router).
    Multiple instances of the link state advertisement may exist in the
    routing domain at the same time.  It is then necessary to determine
    which instance is more recent.  This is accomplished by examining
    the LS age, LS sequence number and LS checksum fields that are also
    contained in the link state advertisement header.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |    Options    |    LS type    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



    LS age
        The time in seconds since the link state advertisement was
        originated.

    Options
        The optional capabilities supported by the described portion of
        the routing domain.  OSPF's optional capabilities are documented
        in Section A.2.

    LS type
        The type of the link state advertisement.  Each link state type
        has a separate advertisement format.  The link state types are
        as follows (see Section 12.1.3 for further explanation):

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                        LS Type   Description
                        ___________________________________
                        1         Router links
                        2         Network links
                        3         Summary link (IP network)
                        4         Summary link (ASBR)
                        5         AS external link




    Link State ID
        This field identifies the portion of the internet environment
        that is being described by the advertisement.  The contents of
        this field depend on the advertisement's LS type.  For example,
        in network links advertisements the Link State ID is set to the
        IP interface address of the network's Designated Router (from
        which the network's IP address can be derived).  The Link State
        ID is further discussed in Section 12.1.4.

    Advertising Router
        The Router ID of the router that originated the link state
        advertisement.  For example, in network links advertisements
        this field is set to the Router ID of the network's Designated
        Router.

    LS sequence number
        Detects old or duplicate link state advertisements.  Successive
        instances of a link state advertisement are given successive LS
        sequence numbers.  See Section 12.1.6 for more details.

    LS checksum
        The Fletcher checksum of the complete contents of the link state
        advertisement, including the link state advertisement header but
        excepting the LS age field. See Section 12.1.7 for more details.

    length
        The length in bytes of the link state advertisement.  This
        includes the 20 byte link state advertisement header.

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A.4.2 Router links advertisements

    Router links advertisements are the Type 1 link state
    advertisements.  Each router in an area originates a router links
    advertisement.  The advertisement describes the state and cost of
    the router's links (i.e., interfaces) to the area.  All of the
    router's links to the area must be described in a single router
    links advertisement.  For details concerning the construction of
    router links advertisements, see Section 12.4.1.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |       1       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    0    |V|E|B|        0      |            # links            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Link ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Link Data                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     # TOS     |        TOS 0 metric           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      TOS      |        0      |            metric             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      TOS      |        0      |            metric             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Link ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Link Data                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |


    In router links advertisements, the Link State ID field is set to
    the router's OSPF Router ID.  The T-bit is set in the
    advertisement's Option field if and only if the router is able to

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    calculate a separate set of routes for each IP TOS.  Router links
    advertisements are flooded throughout a single area only.

    bit V
        When set, the router is an endpoint of an active virtual link
        that is using the described area as a Transit area (V is for
        virtual link endpoint).

    bit E
        When set, the router is an AS boundary router (E is for
        external)

    bit B
        When set, the router is an area border router (B is for border)

    # links
        The number of router links described by this advertisement.
        This must be the total collection of router links (i.e.,
        interfaces) to the area.


    The following fields are used to describe each router link (i.e.,
    interface). Each router link is typed (see the below Type field).
    The Type field indicates the kind of link being described.  It may
    be a link to a transit network, to another router or to a stub
    network.  The values of all the other fields describing a router
    link depend on the link's Type.  For example, each link has an
    associated 32-bit data field.  For links to stub networks this field
    specifies the network's IP address mask.  For other link types the
    Link Data specifies the router's associated IP interface address.


    Type
        A quick description of the router link.  One of the following.
        Note that host routes are classified as links to stub networks
        whose network mask is 0xffffffff.



                 Type   Description
                 __________________________________________________
                 1      Point-to-point connection to another router
                 2      Connection to a transit network
                 3      Connection to a stub network
                 4      Virtual link

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    Link ID
        Identifies the object that this router link connects to.  Value
        depends on the link's Type.  When connecting to an object that
        also originates a link state advertisement (i.e., another router
        or a transit network) the Link ID is equal to the neighboring
        advertisement's Link State ID.  This provides the key for
        looking up said advertisement in the link state database.  See
        Section 12.2 for more details.



                       Type   Link ID
                       ______________________________________
                       1      Neighboring router's Router ID
                       2      IP address of Designated Router
                       3      IP network/subnet number
                       4      Neighboring router's Router ID




    Link Data
        Contents again depend on the link's Type field. For connections
        to stub networks, it specifies the network's IP address mask.
        For unnumbered point-to-point connections, it specifies the
        interface's MIB-II [RFC 1213] ifIndex value. For the other link
        types it specifies the router's associated IP interface address.
        This latter piece of information is needed during the routing
        table build process, when calculating the IP address of the next
        hop. See Section 16.1.1 for more details.

    # TOS
        The number of different TOS metrics given for this link, not
        counting the required metric for TOS 0.  For example, if no
        additional TOS metrics are given, this field should be set to 0.

    TOS 0 metric
        The cost of using this router link for TOS 0.


    For each link, separate metrics may be specified for each Type of
    Service (TOS).  The metric for TOS 0 must always be included, and
    was discussed above.  Metrics for non-zero TOS are described below.
    The encoding of TOS in OSPF link state advertisements is described
    in Section 12.3.  Note that the cost for non-zero TOS values that
    are not specified defaults to the TOS 0 cost.  Metrics must be
    listed in order of increasing TOS encoding.  For example, the metric
    for TOS 16 must always follow the metric for TOS 8 when both are

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    specified.


    TOS IP Type of Service that this metric refers to.  The encoding of
        TOS in OSPF link state advertisements is described in Section
        12.3.

    metric
        The cost of using this outbound router link, for traffic of the
        specified TOS.

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A.4.3 Network links advertisements

    Network links advertisements are the Type 2 link state
    advertisements.  A network links advertisement is originated for
    each transit network in the area.  A transit network is a multi-
    access network that has more than one attached router.  The network
    links advertisement is originated by the network's Designated
    Router.  The advertisement describes all routers attached to the
    network, including the Designated Router itself.  The
    advertisement's Link State ID field lists the IP interface address
    of the Designated Router.

    The distance from the network to all attached routers is zero, for
    all Types of Service.  This is why the TOS and metric fields need
    not be specified in the network links advertisement.  For details
    concerning the construction of network links advertisements, see
    Section 12.4.2.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |      Options  |      2        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Network Mask                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Attached Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |



    Network Mask
        The IP address mask for the network.  For example, a class A
        network would have the mask 0xff000000.

    Attached Router
        The Router IDs of each of the routers attached to the network.
        Actually, only those routers that are fully adjacent to the
        Designated Router are listed.  The Designated Router includes

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        itself in this list.  The number of routers included can be
        deduced from the link state advertisement header's length field.

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A.4.4 Summary link advertisements

    Summary link advertisements are the Type 3 and 4 link state
    advertisements.  These advertisements are originated by area border
    routers.  A separate summary link advertisement is made for each
    destination (known to the router) which belongs to the AS, yet is
    outside the area.  For details concerning the construction of
    summary link advertisements, see Section 12.4.3.

    Type 3 link state advertisements are used when the destination is an
    IP network.  In this case the advertisement's Link State ID field is
    an IP network number (if necessary, the Link State ID can also have
    one or more of the network's "host" bits set; see Appendix F for
    details). When the destination is an AS boundary router, a Type 4
    advertisement is used, and the Link State ID field is the AS
    boundary router's OSPF Router ID.  (To see why it is necessary to
    advertise the location of each ASBR, consult Section 16.4.)  Other
    than the difference in the Link State ID field, the format of Type 3
    and 4 link state advertisements is identical.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |    3 or 4     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Network Mask                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     TOS       |                  metric                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |


    For stub areas, Type 3 summary link advertisements can also be used
    to describe a (per-area) default route.  Default summary routes are
    used in stub areas instead of flooding a complete set of external
    routes.  When describing a default summary route, the
    advertisement's Link State ID is always set to DefaultDestination
    (0.0.0.0) and the Network Mask is set to 0.0.0.0.

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    Separate costs may be advertised for each IP Type of Service.  The
    encoding of TOS in OSPF link state advertisements is described in
    Section 12.3.  Note that the cost for TOS 0 must be included, and is
    always listed first.  If the T-bit is reset in the advertisement's
    Option field, only a route for TOS 0 is described by the
    advertisement.  Otherwise, routes for the other TOS values are also
    described; if a cost for a certain TOS is not included, its cost
    defaults to that specified for TOS 0.

    Network Mask
        For Type 3 link state advertisements, this indicates the
        destination network's IP address mask.  For example, when
        advertising the location of a class A network the value
        0xff000000 would be used.  This field is not meaningful and must
        be zero for Type 4 link state advertisements.


    For each specified Type of Service, the following fields are
    defined.  The number of TOS routes included can be calculated from
    the link state advertisement header's length field.  Values for TOS
    0 must be specified; they are listed first.  Other values must be
    listed in order of increasing TOS encoding.  For example, the cost
    for TOS 16 must always follow the cost for TOS 8 when both are
    specified.


    TOS The Type of Service that the following cost concerns.  The
        encoding of TOS in OSPF link state advertisements is described
        in Section 12.3.

    metric
        The cost of this route.  Expressed in the same units as the
        interface costs in the router links advertisements.

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A.4.5 AS external link advertisements

    AS external link advertisements are the Type 5 link state
    advertisements.  These advertisements are originated by AS boundary
    routers.  A separate advertisement is made for each destination
    (known to the router) which is external to the AS.  For details
    concerning the construction of AS external link advertisements, see
    Section 12.4.3.

    AS external link advertisements usually describe a particular
    external destination.  For these advertisements the Link State ID
    field specifies an IP network number (if necessary, the Link State
    ID can also have one or more of the network's "host" bits set; see
    Appendix F for details).  AS external link advertisements are also
    used to describe a default route.  Default routes are used when no
    specific route exists to the destination.  When describing a default
    route, the Link State ID is always set to DefaultDestination
    (0.0.0.0) and the Network Mask is set to 0.0.0.0.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |      5        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Network Mask                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |E|    TOS      |                  metric                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Forwarding address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      External Route Tag                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |



    Separate costs may be advertised for each IP Type of Service.  The
    encoding of TOS in OSPF link state advertisements is described in
    Section 12.3.  Note that the cost for TOS 0 must be included, and is

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    always listed first.  If the T-bit is reset in the advertisement's
    Option field, only a route for TOS 0 is described by the
    advertisement.  Otherwise, routes for the other TOS values are also
    described; if a cost for a certain TOS is not included, its cost
    defaults to that specified for TOS 0.

    Network Mask
        The IP address mask for the advertised destination.  For
        example, when advertising a class A network the mask 0xff000000
        would be used.


    For each specified Type of Service, the following fields are
    defined.  The number of TOS routes included can be calculated from
    the link state advertisement header's length field.  Values for TOS
    0 must be specified; they are listed first.  Other values must be
    listed in order of increasing TOS encoding.  For example, the cost
    for TOS 16 must always follow the cost for TOS 8 when both are
    specified.


    bit E
        The type of external metric.  If bit E is set, the metric
        specified is a Type 2 external metric.  This means the metric is
        considered larger than any link state path.  If bit E is zero,
        the specified metric is a Type 1 external metric.  This means
        that is is comparable directly (without translation) to the link
        state metric.

    Forwarding address
        Data traffic for the advertised destination will be forwarded to
        this address.  If the Forwarding address is set to 0.0.0.0, data
        traffic will be forwarded instead to the advertisement's
        originator (i.e., the responsible AS boundary router).

    TOS The Type of Service that the following cost concerns.  The
        encoding of TOS in OSPF link state advertisements is described
        in Section 12.3.

    metric
        The cost of this route.  Interpretation depends on the external
        type indication (bit E above).

    External Route Tag
        A 32-bit field attached to each external route.  This is not
        used by the OSPF protocol itself.  It may be used to communicate
        information between AS boundary routers; the precise nature of
        such information is outside the scope of this specification.

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B. Architectural Constants

    Several OSPF protocol parameters have fixed architectural values.
    These parameters have been referred to in the text by names such as
    LSRefreshTime.  The same naming convention is used for the
    configurable protocol parameters.  They are defined in Appendix C.

    The name of each architectural constant follows, together with its
    value and a short description of its function.


    LSRefreshTime
        The maximum time between distinct originations of any particular
        link state advertisement.  When the LS age field of one of the
        router's self-originated advertisements reaches the value
        LSRefreshTime, a new instance of the link state advertisement is
        originated, even though the contents of the advertisement (apart
        from the link state header) will be the same.  The value of
        LSRefreshTime is set to 30 minutes.

    MinLSInterval
        The minimum time between distinct originations of any particular
        link state advertisement.  The value of MinLSInterval is set to
        5 seconds.

    MaxAge
        The maximum age that a link state advertisement can attain. When
        an advertisement's LS age field reaches MaxAge, it is reflooded
        in an attempt to flush the advertisement from the routing domain
        (See Section 14). Advertisements of age MaxAge are not used in
        the routing table calculation.  The value of MaxAge must be
        greater than LSRefreshTime.  The value of MaxAge is set to 1
        hour.

    CheckAge
        When the age of a link state advertisement (that is contained in
        the link state database) hits a multiple of CheckAge, the
        advertisement's checksum is verified.  An incorrect checksum at
        this time indicates a serious error.  The value of CheckAge is
        set to 5 minutes.

    MaxAgeDiff
        The maximum time dispersion that can occur, as a link state
        advertisement is flooded throughout the AS.  Most of this time
        is accounted for by the link state advertisements sitting on
        router output queues (and therefore not aging) during the
        flooding process.  The value of MaxAgeDiff is set to 15 minutes.

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    LSInfinity
        The metric value indicating that the destination described by a
        link state advertisement is unreachable. Used in summary link
        advertisements and AS external link advertisements as an
        alternative to premature aging (see Section 14.1). It is defined
        to be the 24-bit binary value of all ones: 0xffffff.

    DefaultDestination
        The Destination ID that indicates the default route.  This route
        is used when no other matching routing table entry can be found.
        The default destination can only be advertised in AS external
        link advertisements and in stub areas' type 3 summary link
        advertisements.  Its value is the IP address 0.0.0.0.


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