Connected: An Internet Encyclopedia
2. The Topological Database

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2. The Topological Database

2. The Topological Database

The Autonomous System's topological 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.

The vertices of the graph can be further typed according to function. Only some of these types carry transit data traffic; that is, traffic that is neither locally originated nor locally destined. Vertices that can carry transit traffic are indicated on the graph by having both incoming and outgoing edges.

                     Vertex type   Vertex name    Transit?
                     1             Router         yes
                     2             Network        yes
                     3             Stub network   no

                          Table 1: OSPF vertex types.

OSPF supports the following types of physical networks:

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.

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 protocol makes further use of multicast capabilities, if they exist. 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 also discovered on these nets using OSPF's Hello Protocol. However, due to the lack of broadcast capability, some configuration information is necessary for the correct operation of the Hello Protocol. On these 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.

The neighborhood of each network node in the graph depends on whether the network has multi-access capabilities (either broadcast or non-broadcast) and, if so, the number of routers having an interface to the network. The three cases are depicted in Figure 1. Rectangles indicate routers. Circles and oblongs indicate multi- access 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 a network with its connected routers, with the resulting graph shown on the right.

Two routers joined by a point-to-point network are represented in the directed graph as being directly connected by a pair of edges, one in each direction. Interfaces to physical point-to-point networks need not be assigned IP addresses. Such a point-to-point network is called unnumbered. The graphical representation of point-to-point networks is designed so that unnumbered networks can be supported naturally. When interface addresses exist, they are modelled as stub routes. Note that each router would then have a stub connection to the other router's interface address (see Figure 1).

When multiple routers are attached to a multi-access network, the directed graph shows all routers bidirectionally connected to the network vertex (again, see Figure 1). If only a single router is attached to a multi-access network, the network will appear in the directed graph as a stub connection.


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

                     Physical point-to-point 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 |   |
                +---+      +---+

                          Multi-access networks

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

                       Stub multi-access networks

                    Figure 1: 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.

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.

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 EGP connections to other Autonomous Systems. A set of EGP-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 EGP-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.

The topological database (or what has been referred to above as the directed graph) is pieced together from link state advertisements generated by the routers. The neighborhood of each transit vertex is represented in a single, separate link state advertisement. Figure 4 shows graphically the link state representation of the two kinds of transit vertices: routers and multi-access networks. Router RT12 has an interface to two broadcast networks and a SLIP line to a host. Network N6 is a broadcast network with three attached routers. The cost of all links from Network N6 to its attached routers is 0. Note that the link state advertisement for Network N6 is actually generated by one of the attached routers: the router that has been elected Designated Router for the network.

Next: 2.1. The shortest-path tree

Connected: An Internet Encyclopedia
2. The Topological Database