rfc2328.txt
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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. 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 costMoy Standards Track [Page 23] RFC 2328 OSPF Version 2 April 1998 (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. 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: Destination Next Hop Distance __________________________________ N12 RT10 10 N13 RT5 14 N14 RT5 14 N15 RT10 17 Table 3: The portion of Router RT6's routing table listing external destinations. Processing of Type 2 external metrics is simpler. The AS boundary router advertising the smallest external metric isMoy Standards Track [Page 24] RFC 2328 OSPF Version 2 April 1998 chosen, regardless of the internal distance to the AS boundary router. Suppose in our example both Router RT5 and Router RT7 were advertising Type 2 external routes. Then all traffic destined for Network N12 would be forwarded to Router RT7, since 2 < 8. When several equal-cost Type 2 routes exist, the internal distance to the advertising routers is used to break the tie. Both Type 1 and Type 2 external metrics can be present in the AS at the same time. In that event, Type 1 external metrics always take precedence. This section has assumed that packets destined for external destinations are always routed through the advertising AS boundary router. This is not always desirable. For example, suppose in Figure 2 there is an additional router attached to Network N6, called Router RTX. Suppose further that RTX does not participate in OSPF routing, but does exchange BGP information with the AS boundary router RT7. Then, Router RT7 would end up advertising OSPF external routes for all destinations that should be routed to RTX. An extra hop will sometimes be introduced if packets for these destinations need always be routed first to Router RT7 (the advertising router). To deal with this situation, the OSPF protocol allows an AS boundary router to specify a "forwarding address" in its AS- external-LSAs. In the above example, Router RT7 would specify RTX's IP address as the "forwarding address" for all those destinations whose packets should be routed directly to RTX. The "forwarding address" has one other application. It enables routers in the Autonomous System's interior to function as "route servers". For example, in Figure 2 the router RT6 could become a route server, gaining external routing information through a combination of static configuration and external routing protocols. RT6 would then start advertising itself as an AS boundary router, and would originate a collection of OSPF AS-external-LSAs. In each AS-external-LSA, Router RT6 would specify the correct Autonomous System exit point to use for the destination through appropriate setting of the LSA's "forwarding address" field.Moy Standards Track [Page 25] RFC 2328 OSPF Version 2 April 1998 2.4. Equal-cost multipath The above discussion has been simplified by considering only a single route to any destination. In reality, if multiple equal-cost routes to a destination exist, they are all discovered and used. This requires no conceptual changes to the algorithm, and its discussion is postponed until we consider the tree-building process in more detail. With equal cost multipath, a router potentially has several available next hops towards any given destination.3. Splitting the AS into Areas OSPF allows collections of contiguous networks and hosts to be grouped together. Such a group, together with the routers having interfaces to any one of the included networks, is called an area. Each area runs a separate copy of the basic link-state routing algorithm. This means that each area has its own link-state database and corresponding graph, as explained in the previous section. The topology of an area is invisible from the outside of the area. Conversely, routers internal to a given area know nothing of the detailed topology external to the area. This isolation of knowledge enables the protocol to effect a marked reduction in routing traffic as compared to treating the entire Autonomous System as a single link-state domain. With the introduction of areas, it is no longer true that all routers in the AS have an identical link-state database. A router actually has a separate link-state database for each area it is connected to. (Routers connected to multiple areas are called area border routers). Two routers belonging to the same area have, for that area, identical area link-state databases. Routing in the Autonomous System takes place on two levels, depending on whether the source and destination of a packet reside in the same area (intra-area routing is used) or different areas (inter-area routing is used). In intra-area routing, the packet is routed solely on information obtained within the area; no routingMoy Standards Track [Page 26] RFC 2328 OSPF Version 2 April 1998 information obtained from outside the area can be used. This protects intra-area routing from the injection of bad routing information. We discuss inter-area routing in Section 3.2. 3.1. The backbone of the Autonomous System The OSPF backbone is the special OSPF Area 0 (often written as Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP addresses). The OSPF backbone always contains all area border routers. The backbone is responsible for distributing routing information between non-backbone areas. The backbone must be contiguous. However, it need not be physically contiguous; backbone connectivity can be established/maintained through the configuration of virtual links. Virtual links can be configured between any two backbone routers that have an interface to a common non-backbone area. Virtual links belong to the backbone. The protocol treats two routers joined by a virtual link as if they were connected by an unnumbered point-to-point backbone network. On the graph of the backbone, two such routers are joined by arcs whose costs are the intra-area distances between the two routers. The routing protocol traffic that flows along the virtual link uses intra- area routing only. 3.2. Inter-area routing When routing a packet between two non-backbone areas the backbone is used. The path that the packet will travel can be broken up into three contiguous pieces: an intra-area path from the source to an area border router, a backbone path between the source and destination areas, and then another intra-area path to the destination. The algorithm finds the set of such paths that have the smallest cost. Looking at this another way, inter-area routing can be pictured as forcing a star configuration on the Autonomous System, with the backbone as hub and each of the non-backbone areas as spokes.Moy Standards Track [Page 27] RFC 2328 OSPF Version 2 April 1998 The topology of the backbone dictates the backbone paths used between areas. The topology of the backbone can be enhanced by adding virtual links. This gives the system administrator some control over the routes taken by inter-area traffic. The correct area border router to use as the packet exits the source area is chosen in exactly the same way routers advertising external routes are chosen. Each area border router in an area summarizes for the area its cost to all networks external to the area. After the SPF tree is calculated for the area, routes to all inter-area destinations are calculated by examining the summaries of the area border routers. 3.3. Classification of routers Before the introduction of areas, the only OSPF routers having a specialized function were those advertising external routing information, such as Router RT5 in Figure 2. When the AS is split into OSPF areas, the routers are further divided according to function into the following four overlapping categories: Internal routers A router with all directl⌨️ 快捷键说明
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