rfc2178.txt

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RFC 2178                     OSPF Version 2                    July 1997


   All routers run the exact same algorithm, in parallel. From the
   link-state database, each router constructs a tree of shortest paths
   with itself as root.  This shortest-path tree gives the route to each
   destination in the Autonomous System.  Externally derived routing
   information appears on the tree as leaves.

   When several equal-cost routes to a destination exist, traffic is
   distributed equally among them.  The cost of a route is described by
   a single dimensionless metric.

   OSPF allows sets of networks to be grouped together.  Such a grouping
   is called an area.  The topology of an area is hidden from the rest
   of the Autonomous System.  This information hiding enables a
   significant reduction in routing traffic.  Also, routing within the
   area is determined only by the area's own topology, lending the area
   protection from bad routing data.  An area is a generalization of an
   IP subnetted network.

   OSPF enables the flexible configuration of IP subnets.  Each route
   distributed by OSPF has a destination and mask.  Two different
   subnets of the same IP network number may have different sizes (i.e.,
   different masks).  This is commonly referred to as variable length
   subnetting.  A packet is routed to the best (i.e., longest or most
   specific) match.  Host routes are considered to be subnets whose
   masks are "all ones" (0xffffffff).

   All OSPF protocol exchanges are authenticated.  This means that only
   trusted routers can participate in the Autonomous System's routing.
   A variety of authentication schemes can be used; in fact, separate
   authentication schemes can be configured for each IP subnet.

   Externally derived routing data (e.g., routes learned from an
   Exterior Gateway Protocol such as BGP; see [Ref23]) is advertised
   throughout the Autonomous System.  This externally derived data is
   kept separate from the OSPF protocol's link state data.  Each
   external route can also be tagged by the advertising router, enabling
   the passing of additional information between routers on the boundary
   of the Autonomous System.

1.2.  Definitions of commonly used terms

   This section provides definitions for terms that have a specific
   meaning to the OSPF protocol and that are used throughout the text.
   The reader unfamiliar with the Internet Protocol Suite is referred to
   [Ref13] for an introduction to IP.






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   Router
      A level three Internet Protocol packet switch.  Formerly called a
      gateway in much of the IP literature.

   Autonomous System
      A group of routers exchanging routing information via a common
      routing protocol.  Abbreviated as AS.

   Interior Gateway Protocol
      The routing protocol spoken by the routers belonging to an
      Autonomous system. Abbreviated as IGP.  Each Autonomous System has
      a single IGP.  Separate Autonomous Systems may be running
      different IGPs.

   Router ID
      A 32-bit number assigned to each router running the OSPF protocol.
      This number uniquely identifies the router within an Autonomous
      System.

   Network
      In this memo, an IP network/subnet/supernet.  It is possible for
      one physical network to be assigned multiple IP network/subnet
      numbers.  We consider these to be separate networks.  Point-to-
      point physical networks are an exception - they are considered a
      single network no matter how many (if any at all) IP
      network/subnet numbers are assigned to them.

   Network mask
      A 32-bit number indicating the range of IP addresses residing on a
      single IP network/subnet/supernet.  This specification displays
      network masks as hexadecimal numbers.  For example, the network
      mask for a class C IP network is displayed as 0xffffff00.  Such a
      mask is often displayed elsewhere in the literature as
      255.255.255.0.

   Point-to-point networks
      A network that joins a single pair of routers.  A 56Kb serial line
      is an example of a point-to-point network.













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   Broadcast networks
      Networks supporting many (more than two) attached routers,
      together with the capability to address a single physical message
      to all of the attached routers (broadcast).  Neighboring routers
      are discovered dynamically on these nets using OSPF's Hello
      Protocol.  The Hello Protocol itself takes advantage of the
      broadcast capability.  The OSPF protocol makes further use of
      multicast capabilities, if they exist.  Each pair of routers on a
      broadcast network is assumed to be able to communicate directly.
      An ethernet is an example of a broadcast network.

   Non-broadcast networks
      Networks supporting many (more than two) routers, but having no
      broadcast capability.  Neighboring routers are maintained on these
      nets using OSPF's Hello Protocol. However, due to the lack of
      broadcast capability, some configuration information may be
      necessary to aid in the discovery of neighbors. On non-broadcast
      networks, OSPF protocol packets that are normally multicast need
      to be sent to each neighboring router, in turn. An X.25 Public
      Data Network (PDN) is an example of a non-broadcast network.

      OSPF runs in one of two modes over non-broadcast networks.  The
      first mode, called non-broadcast multi-access or NBMA, simulates
      the operation of OSPF on a broadcast network. The second mode,
      called Point-to-MultiPoint, treats the non-broadcast network as a
      collection of point-to-point links.  Non-broadcast networks are
      referred to as NBMA networks or Point-to-MultiPoint networks,
      depending on OSPF's mode of operation over the network.

   Interface
      The connection between a router and one of its attached networks.
      An interface has state information associated with it, which is
      obtained from the underlying lower level protocols and the routing
      protocol itself.  An interface to a network has associated with it
      a single IP address and mask (unless the network is an unnumbered
      point-to-point network).  An interface is sometimes also referred
      to as a link.

   Neighboring routers
      Two routers that have interfaces to a common network.  Neighbor
      relationships are maintained by, and usually dynamically
      discovered by, OSPF's Hello Protocol.

   Adjacency
      A relationship formed between selected neighboring routers for the
      purpose of exchanging routing information.  Not every pair of
      neighboring routers become adjacent.




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   Link state advertisement
      Unit of data describing the local state of a router or network.
      For a router, this includes the state of the router's interfaces
      and adjacencies.  Each link state advertisement is flooded
      throughout the routing domain. The collected link state
      advertisements of all routers and networks forms the protocol's
      link state database.  Throughout this memo, link state
      advertisement is abbreviated as LSA.

   Hello Protocol
      The part of the OSPF protocol used to establish and maintain
      neighbor relationships.  On broadcast networks the Hello Protocol
      can also dynamically discover neighboring routers.

   Flooding
      The part of the OSPF protocol that distributes and synchronizes
      the link-state database between OSPF routers.

   Designated Router
      Each broadcast and NBMA network that has at least two attached
      routers has a Designated Router.  The Designated Router generates
      an LSA for the network and has other special responsibilities in
      the running of the protocol.  The Designated Router is elected by
      the Hello Protocol.

      The Designated Router concept enables a reduction in the number of
      adjacencies required on a broadcast or NBMA network.  This in turn
      reduces the amount of routing protocol traffic and the size of the
      link-state database.

   Lower-level protocols
      The underlying network access protocols that provide services to
      the Internet Protocol and in turn the OSPF protocol.  Examples of
      these are the X.25 packet and frame levels for X.25 PDNs, and the
      ethernet data link layer for ethernets.

1.3.  Brief history of link-state routing technology

   OSPF is a link state routing protocol.  Such protocols are also
   referred to in the literature as SPF-based or distributed-database
   protocols.  This section gives a brief description of the
   developments in link-state technology that have influenced the OSPF
   protocol.

   The first link-state routing protocol was developed for use in the
   ARPANET packet switching network.  This protocol is described in
   [Ref3].  It has formed the starting point for all other link-state
   protocols.  The homogeneous ARPANET environment, i.e., single-vendor



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   packet switches connected by synchronous serial lines, simplified the
   design and implementation of the original protocol.

   Modifications to this protocol were proposed in [Ref4].  These
   modifications dealt with increasing the fault tolerance of the
   routing protocol through, among other things, adding a checksum to
   the LSAs (thereby detecting database corruption).  The paper also
   included means for reducing the routing traffic overhead in a link-
   state protocol.  This was accomplished by introducing mechanisms
   which enabled the interval between LSA originations to be increased
   by an order of magnitude.

   A link-state algorithm has also been proposed for use as an ISO IS-IS
   routing protocol.  This protocol is described in [Ref2].  The
   protocol includes methods for data and routing traffic reduction when
   operating over broadcast networks.  This is accomplished by election
   of a Designated Router for each broadcast network, which then
   originates an LSA for the network.

   The OSPF Working Group of the IETF has extended this work in
   developing the OSPF protocol.  The Designated Router concept has been
   greatly enhanced to further reduce the amount of routing traffic
   required.  Multicast capabilities are utilized for additional routing
   bandwidth reduction.  An area routing scheme has been developed
   enabling information hiding/protection/reduction.  Finally, the
   algorithms have been tailored for efficient operation in TCP/IP
   internets.

1.4.  Organization of this document

   The first three sections of this specification give a general
   overview of the protocol's capabilities and functions.  Sections 4-16
   explain the protocol's mechanisms in detail.  Packet formats,
   protocol constants and configuration items are specified in the
   appendices.

   Labels such as HelloInterval encountered in the text refer to
   protocol constants.  They may or may not be configurable.
   Architectural constants are summarized in Appendix B.  Configurable
   constants are summarized in Appendix C.

   The detailed specification of the protocol is presented in terms of
   data structures.  This is done in order to make the explanation more
   precise.  Implementations of the protocol are required to support the
   functionality described, but need not use the precise data structures
   that appear in this memo.





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RFC 2178                     OSPF Version 2                    July 1997