📄 rfc2178.txt
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+ |RT3|--------------|RT6| | +---+ +---+ | |2 Ia|7 | | | | +---------+ | | N4 | | | | | | N11 | | +---------+ | | | | | N12 |3 | |6 2/ +---+ | +---+/ |RT9| | |RT7|---N15 +---+ | +---+ 9 |1 + | |1 _|__ | Ib|5 __|_ / \ 1+----+2 | 3+----+1 / \ * N9 *------|RT11|----|---|RT10|---* N6 * \____/ +----+ | +----+ \____/ | | | |1 + |1 +--+ 10+----+ N8 +---+ |H1|-----|RT12| |RT8| +--+SLIP +----+ +---+ |2 |4 | | +---------+ +--------+ N10 N7 Figure 2: A sample Autonomous SystemMoy Standards Track [Page 16] RFC 2178 OSPF Version 2 July 1997 **FROM** |RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT| |1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9| ----- --------------------------------------------- RT1| | | | | | | | | | | | |0 | | | | RT2| | | | | | | | | | | | |0 | | | | RT3| | | | | |6 | | | | | | |0 | | | | RT4| | | | |8 | | | | | | | |0 | | | | RT5| | | |8 | |6 |6 | | | | | | | | | | RT6| | |8 | |7 | | | | |5 | | | | | | | RT7| | | | |6 | | | | | | | | |0 | | | * RT8| | | | | | | | | | | | | |0 | | | * RT9| | | | | | | | | | | | | | | |0 | T RT10| | | | | |7 | | | | | | | |0 |0 | | O RT11| | | | | | | | | | | | | | |0 |0 | * RT12| | | | | | | | | | | | | | | |0 | * N1|3 | | | | | | | | | | | | | | | | N2| |3 | | | | | | | | | | | | | | | N3|1 |1 |1 |1 | | | | | | | | | | | | | N4| | |2 | | | | | | | | | | | | | | N6| | | | | | |1 |1 | |1 | | | | | | | N7| | | | | | | |4 | | | | | | | | | N8| | | | | | | | | |3 |2 | | | | | | N9| | | | | | | | |1 | |1 |1 | | | | | N10| | | | | | | | | | | |2 | | | | | N11| | | | | | | | |3 | | | | | | | | N12| | | | |8 | |2 | | | | | | | | | | N13| | | | |8 | | | | | | | | | | | | N14| | | | |8 | | | | | | | | | | | | N15| | | | | | |9 | | | | | | | | | | H1| | | | | | | | | | | |10| | | | | Figure 3: The resulting directed graph Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X. The link-state database is pieced together from LSAs generated by the routers. In the associated graphical representation, the neighborhood of each router or transit network is represented in a single, separate LSA. Figure 4 shows these LSAs graphically. 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.Moy Standards Track [Page 17] RFC 2178 OSPF Version 2 July 1997 Note that the LSA for Network N6 is actually generated by one of the network's attached routers: the router that has been elected Designated Router for the network.2.2. The shortest-path tree When no OSPF areas are configured, each router in the Autonomous System has an identical link-state database, leading to an identical graphical representation. A router generates its routing table from this graph by calculating a tree of shortest paths with the router itself as root. Obviously, the shortest- path tree depends on the router doing the calculation. The shortest-path tree for Router RT6 in our example is depicted in Figure 5. The tree gives the entire path to any destination network or host. However, only the next hop to the destination is used in the forwarding process. Note also that the best route to any router has also been calculated. For the processing of external data, we note the next hop and distance to any router advertising external routes. The resulting routing table for Router RT6 is pictured in Table 2. Note that there is a separate route for each end of a numbered point-to-point network (in this case, the serial line between Routers RT6 and RT10). **FROM** **FROM** |RT12|N9|N10|H1| |RT9|RT11|RT12|N9| * -------------------- * ---------------------- * RT12| | | | | * RT9| | | |0 | T N9|1 | | | | T RT11| | | |0 | O N10|2 | | | | O RT12| | | |0 | * H1|10 | | | | * N9| | | | | * * RT12's router-LSA N9's network-LSA Figure 4: Individual link state components Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X.Moy Standards Track [Page 18] RFC 2178 OSPF Version 2 July 1997 RT6(origin) RT5 o------------o-----------o Ib /|\ 6 |\ 7 8/8|8\ | \ / | \ 6| \ o | o | \7 N12 o N14 | \ N13 2 | \ N4 o-----o RT3 \ / \ 5 1/ RT10 o-------o Ia / |\ RT4 o-----o N3 3| \1 /| | \ N6 RT7 / | N8 o o---------o / | | | /| RT2 o o RT1 | | 2/ |9 / | | |RT8 / | /3 |3 RT11 o o o o / | | | N12 N15 N2 o o N1 1| |4 | | N9 o o N7 /| / | N11 RT9 / |RT12 o--------o-------o o--------o H1 3 | 10 |2 | o N10 Figure 5: The SPF tree for Router RT6 Edges that are not marked with a cost have a cost of of zero (these are network-to-router links). Routes to networks N12-N15 are external information that is considered in Section 2.3Moy Standards Track [Page 19] RFC 2178 OSPF Version 2 July 1997 Destination Next Hop Distance __________________________________ N1 RT3 10 N2 RT3 10 N3 RT3 7 N4 RT3 8 Ib * 7 Ia RT10 12 N6 RT10 8 N7 RT10 12 N8 RT10 10 N9 RT10 11 N10 RT10 13 N11 RT10 14 H1 RT10 21 __________________________________ RT5 RT5 6 RT7 RT10 8 Table 2: The portion of Router RT6's routing table listing local destinations. Routes to networks belonging to other AS'es (such as N12) appear as dashed lines on the shortest path tree in Figure 5. Use of this externally derived routing information is considered in the next section.2.3. Use of external routing information After the tree is created the external routing information is examined. This external routing information may originate from another routing protocol such as BGP, or be statically configured (static routes). Default routes can also be included as part of the Autonomous System's external routing information. External routing information is flooded unaltered throughout the AS. In our example, all the routers in the Autonomous System know that Router RT7 has two external routes, with metrics 2 and 9. OSPF supports two types of external metrics. Type 1 external metrics are expressed in the same units as OSPF interface cost (i.e., in terms of the link state metric). Type 2 external metrics are an order of magnitude larger; any Type 2 metric is considered greater than the cost of any path internal to the AS. Use of Type 2 external metrics assumes that routing between AS'es is the major cost of routing a packet, and eliminates the need for conversion of external costs to internal link state metrics.Moy Standards Track [Page 20] RFC 2178 OSPF Version 2 July 1997 As an example of Type 1 external metric processing, suppose that the Routers RT7 and RT5 in Figure 2 are advertising Type 1 external metrics. For each advertised external route, the total cost from Router RT6 is calculated as the sum of the external route's advertised cost and the distance from Router RT6 to the advertising router. When two routers are advertising the same external destination, RT6 picks the advertising router providing the minimum total cost. RT6 then sets the next hop to the external destination equal to the next hop that would be used when routing packets to the chosen advertising router. In Figure 2, both Router RT5 and RT7 are advertising an external route to destination Network N12. Router RT7 is preferred since it is advertising N12 at a distance of 10 (8+2) to Router RT6, which is better than Router RT5's 14 (6+8). Table 3 shows the entries that are added to the routing table when external routes are examined:⌨️ 快捷键说明
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