rfc2178.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

   have given to this project.

   The OSPF Point-to-MultiPoint interface is based on work done by Fred
   Baker.

   The OSPF Cryptographic Authentication option was developed by Fred
   Baker and Ran Atkinson.

2.  The Link-state Database: organization and calculations

   The following subsections describe the organization of OSPF's link-
   state database, and the routing calculations that are performed on
   the database in order to produce a router's routing table.

2.1.  Representation of routers and networks

   The Autonomous System's link-state database describes a directed
   graph.  The vertices of the graph consist of routers and networks.  A
   graph edge connects two routers when they are attached via a physical
   point-to-point network.  An edge connecting a router to a network
   indicates that the router has an interface on the network. Networks
   can be either transit or stub networks. Transit networks are those
   capable of carrying data traffic that is neither locally originated
   nor locally destined. A transit network is represented by a graph
   vertex having both incoming and outgoing edges. A stub network's
   vertex has only incoming edges.

   The neighborhood of each network node in the graph depends on the
   network's type (point-to-point, broadcast, NBMA or Point-to-
   MultiPoint) and the number of routers having an interface to the
   network.  Three cases are depicted in Figure 1a.  Rectangles indicate
   routers.  Circles and oblongs indicate networks.  Router names are
   prefixed with the letters RT and network names with the letter N.
   Router interface names are prefixed by the letter I.  Lines between
   routers indicate point-to-point networks.  The left side of the
   figure shows networks with their connected routers, with the
   resulting graphs shown on the right.








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                                                  **FROM**

                                           *      |RT1|RT2|
                +---+Ia    +---+           *   ------------
                |RT1|------|RT2|           T   RT1|   | X |
                +---+    Ib+---+           O   RT2| X |   |
                                           *    Ia|   | X |
                                           *    Ib| X |   |

                    Physical point-to-point networks

                                                  **FROM**
                      +---+                *
                      |RT7|                *      |RT7| N3|
                      +---+                T   ------------
                        |                  O   RT7|   |   |
            +----------------------+       *    N3| X |   |
                       N3                  *

                             Stub networks

                +---+      +---+
                |RT3|      |RT4|              |RT3|RT4|RT5|RT6|N2 |
                +---+      +---+        *  ------------------------
                  |    N2    |          *  RT3|   |   |   |   | X |
            +----------------------+    T  RT4|   |   |   |   | X |
                  |          |          O  RT5|   |   |   |   | X |
                +---+      +---+        *  RT6|   |   |   |   | X |
                |RT5|      |RT6|        *   N2| X | X | X | X |   |
                +---+      +---+

                       Broadcast or NBMA networks

                   Figure 1a: Network map components

   Networks and routers are represented by vertices.  An edge connects
   Vertex A to Vertex B iff the intersection of Column A and Row B is
   marked with an X.

   The top of Figure 1a shows two routers connected by a point-to-point
   link. In the resulting link-state database graph, the two router
   vertices are directly connected by a pair of edges, one in each
   direction. Interfaces to point-to-point networks need not be assigned
   IP addresses.  When interface addresses are assigned, they are
   modelled as stub links, with each router advertising a stub
   connection to the other router's interface address. Optionally, an IP





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   subnet can be assigned to the point-to-point network. In this case,
   both routers advertise a stub link to the IP subnet, instead of
   advertising each others' IP interface addresses.

   The middle of Figure 1a shows a network with only one attached router
   (i.e., a stub network). In this case, the network appears on the end
   of a stub connection in the link-state database's graph.

   When multiple routers are attached to a broadcast network, the link-
   state database graph shows all routers bidirectionally connected to
   the network vertex. This is pictured at the bottom of Figure 1a.

   Each network (stub or transit) in the graph has an IP address and
   associated network mask.  The mask indicates the number of nodes on
   the network.  Hosts attached directly to routers (referred to as host
   routes) appear on the graph as stub networks.  The network mask for a
   host route is always 0xffffffff, which indicates the presence of a
   single node.

2.1.1. Representation of non-broadcast networks

   As mentioned previously, OSPF can run over non-broadcast networks in
   one of two modes: NBMA or Point-to-MultiPoint.  The choice of mode
   determines the way that the Hello protocol and flooding work over the
   non-broadcast network, and the way that the network is represented in
   the link-state database.

   In NBMA mode, OSPF emulates operation over a broadcast network: a
   Designated Router is elected for the NBMA network, and the Designated
   Router originates an LSA for the network. The graph representation
   for broadcast networks and NBMA networks is identical. This
   representation is pictured in the middle of Figure 1a.

   NBMA mode is the most efficient way to run OSPF over non-broadcast
   networks, both in terms of link-state database size and in terms of
   the amount of routing protocol traffic.  However, it has one
   significant restriction: it requires all routers attached to the NBMA
   network to be able to communicate directly. This restriction may be
   met on some non-broadcast networks, such as an ATM subnet utilizing
   SVCs. But it is often not met on other non-broadcast networks, such
   as PVC-only Frame Relay networks. On non-broadcast networks where not
   all routers can communicate directly you can break the non-broadcast
   network into logical subnets, with the routers on each subnet being
   able to communicate directly, and then run each separate subnet as an
   NBMA network (see [Ref15]). This however requires quite a bit of
   administrative overhead, and is prone to misconfiguration. It is
   probably better to run such a non-broadcast network in Point-to-
   Multipoint mode.



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   In Point-to-MultiPoint mode, OSPF treats all router-to-router
   connections over the non-broadcast network as if they were point-to-
   point links. No Designated Router is elected for the network, nor is
   there an LSA generated for the network. In fact, a vertex for the
   Point-to-MultiPoint network does not appear in the graph of the
   link-state database.

   Figure 1b illustrates the link-state database representation of a
   Point-to-MultiPoint network. On the left side of the figure, a
   Point-to-MultiPoint network is pictured. It is assumed that all
   routers can communicate directly, except for routers RT4 and RT5. I3
   though I6 indicate the routers' IP interface addresses on the Point-
   to-MultiPoint network.  In the graphical representation of the link-
   state database, routers that can communicate directly over the
   Point-to-MultiPoint network are joined by bidirectional edges, and
   each router also has a stub connection to its own IP interface
   address (which is in contrast to the representation of real point-
   to-point links; see Figure 1a).

   On some non-broadcast networks, use of Point-to-MultiPoint mode and
   data-link protocols such as Inverse ARP (see [Ref14]) will allow
   autodiscovery of OSPF neighbors even though broadcast support is not
   available.

2.1.2.  An example link-state database

   Figure 2 shows a sample map of an Autonomous System.  The rectangle
   labelled H1 indicates a host, which has a SLIP connection to Router
   RT12. Router RT12 is therefore advertising a host route.  Lines
   between routers indicate physical point-to-point networks.  The only
   point-to-point network that has been assigned interface addresses is
   the one joining Routers RT6 and RT10.  Routers RT5 and RT7 have BGP
   connections to other Autonomous Systems.  A set of BGP-learned routes
   have been displayed for both of these routers.

   A cost is associated with the output side of each router interface.
   This cost is configurable by the system administrator.  The lower the
   cost,the more likely the interface is to be used to forward data
   traffic.  Costs are also associated with the externally derived
   routing data (e.g., the BGP-learned routes).

   The directed graph resulting from the map in Figure 2 is depicted in
   Figure 3.  Arcs are labelled with the cost of the corresponding
   router output interface. Arcs having no labelled cost have a cost of
   0.  Note that arcs leading from networks to routers always have cost
   0; they are significant nonetheless.  Note also that the externally
   derived routing data appears on the graph as stubs.




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                                                  **FROM**
                +---+      +---+
                |RT3|      |RT4|              |RT3|RT4|RT5|RT6|
                +---+      +---+        *  --------------------
                I3|    N2    |I4        *  RT3|   | X | X | X |
            +----------------------+    T  RT4| X |   |   | X |
                I5|          |I6        O  RT5| X |   |   | X |
                +---+      +---+        *  RT6| X | X | X |   |
                |RT5|      |RT6|        *   I3| X |   |   |   |
                +---+      +---+            I4|   | X |   |   |
                                            I5|   |   | X |   |
                                            I6|   |   |   | X |


                   Figure 1b: Network map components
                      Point-to-MultiPoint networks

          All routers can communicate directly over N2, except
             routers RT4 and RT5. I3 through I6 indicate IP
                          interface addresses































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RFC 2178                     OSPF Version 2                    July 1997


                 +
                 | 3+---+                     N12      N14
               N1|--|RT1|\ 1                    \ N13 /
                 |  +---+ \                     8\ |8/8
                 +         \ ____                 \|/
                            /    \   1+---+8    8+---+6
                           *  N3  *---|RT4|------|RT5|--------+
                            \____/    +---+      +---+        |
                  +         /   |                  |7         |
                  | 3+---+ /    |                  |          |
                N2|--|RT2|/1    |1                 |6         |
                  |  +---+    +---+8            6+---+        |