Network Working Group J. Moy Request for Comments: 1247 Proteon, Inc. Obsoletes: RFC 1131 July 1991 OSPF Version 2 Status of this Memo This RFC specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the ``IAB Official Protocol Standards'' for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract This memo documents version 2 of the OSPF protocol. OSPF is a link- state based routing protocol. It is designed to be run internal to a single Autonomous System. Each OSPF router maintains an identical database describing the Autonomous System's topology. From this database, a routing table is calculated by constructing a shortest-path tree. OSPF recalculates routes quickly in the face of topological changes, utilizing a minimum of routing protocol traffic. OSPF provides support for equal-cost multipath. Separate routes can be calculated for each IP type of service. An area routing capability is provided, enabling an additional level of routing protection and a reduction in routing protocol traffic. In addition, all OSPF routing protocol exchanges are authenticated. Version 1 of the OSPF protocol was documented in RFC 1131. The differences between the two versions are explained in Appendix F. Please send comments to firstname.lastname@example.org. 1. Introduction This document is a specification of the Open Shortest Path First (OSPF) internet routing protocol. OSPF is classified as an Internal Gateway Protocol (IGP). This means that it distributes routing information between routers belonging to a single Autonomous System. The OSPF protocol is based on SPF or link-state technology. This is a departure
from the Bellman-Ford base used by traditional internet routing protocols. The OSPF protocol was developed by the OSPF working group of the Internet Engineering Task Force. It has been designed expressly for the internet environment, including explicit support for IP subnetting, TOS-based routing and the tagging of externally-derived routing information. OSPF also provides for the authentication of routing updates, and utilizes IP multicast when sending/receiving the updates. In addition, much work has been done to produce a protocol that responds quickly to topology changes, yet involves small amounts of routing protocol traffic. The author would like to thank Rob Coltun, Milo Medin, Mike Petry and the rest of the OSPF working group for the ideas and support they have given to this project. 1.1 Protocol overview OSPF routes IP packets based solely on the destination IP address and IP Type of Service found in the IP packet header. IP packets are routed "as is" -- they are not encapsulated in any further protocol headers as they transit the Autonomous System. OSPF is a dynamic routing protocol. It quickly detects topological changes in the AS (such as router interface failures) and calculates new loop-free routes after a period of convergence. This period of convergence is short and involves a minimum of routing traffic. In an SPF-based routing protocol, each router maintains a database describing the Autonomous System's topology. Each participating router has an identical database. Each individual piece of this database is a particular router's local state (e.g., the router's usable interfaces and reachable neighbors). The router distributes its local state throughout the Autonomous System by flooding. All routers run the exact same algorithm, in parallel. From the topological 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. OSPF calculates separate routes for each Type of Service (TOS). 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 subnets. 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; a single authentication scheme is configured for each area. This enables some areas to use much stricter authentication than others. Externally derived routing data (e.g., routes learned from the Exterior Gateway Protocol (EGP)) is passed transparently 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 boundaries of the Autonomous System. 1.2 Definitions of commonly used terms Here is a collection of definitions for terms that have a specific meaning to the protocol and that are used throughout the text. The reader unfamiliar with the Internet Protocol Suite is referred to [RS- 85-153] for an introduction to IP. 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. Internal Gateway Protocol The routing protocol spoken by the routers belonging to an Autonomous system. Abbreviated as IGP. Each Autonomous System has
a single IGP. Different 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 paper, an IP network or subnet. 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. 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. Multi-access networks Those physical networks that support the attachment of multiple (more than two) routers. Each pair of routers on such a network is assumed to be able to communicate directly (e.g., multi-drop networks are excluded). 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. On multi- access networks, neighbors are 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.
Link state advertisement Describes to the local state of a router or network. 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 topological database. Hello protocol The part of the OSPF protocol used to establish and maintain neighbor relationships. On multi-access networks the Hello protocol can also dynamically discover neighboring routers. Designated Router Each multi-access network that has at least two attached routers has a Designated Router. The Designated Router generates a link state advertisement for the multi-access 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 multi-access network. This in turn reduces the amount of routing protocol traffic and the size of the topological 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 PDNs, and the ethernet data link layer for ethernets. 1.3 Brief history of SPF-based routing technology OSPF is an SPF-based routing protocol. Such protocols are also referred to in the literature as link-state or distributed-database protocols. This section gives a brief description of the developments in SPF-based technology that have influenced the OSPF protocol. The first SPF-based routing protocol was developed for use in the ARPANET packet switching network. This protocol is described in [McQuillan]. It has formed the starting point for all other SPF-based protocols. The homogeneous Arpanet environment, i.e., single-vendor packet switches connected by synchronous serial lines, simplified the design and implementation of the original protocol. Modifications to this protocol were proposed in [Perlman]. These modifications dealt with increasing the fault tolerance of the routing protocol through, among other things, adding a checksum to the link
state advertisements (thereby detecting database corruption). The paper also included means for reducing the routing traffic overhead in an SPF-based protocol. This was accomplished by introducing mechanisms which enabled the interval between link state advertisements to be increased by an order of magnitude. An SPF-based algorithm has also been proposed for use as an ISO IS-IS routing protocol. This protocol is described in [DEC]. 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 a link state advertisement for the network. The OSPF subcommittee 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 algorithm has been modified for efficient operation in the internet environment. 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, configuration items and required management statistics are specified in the appendices. Labels such as HelloInterval encountered in the text refer to protocol constants. They may or may not be configurable. The architectural constants are explained in Appendix B. The configurable constants are explained 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 paper. 2. The Topological Database The database of the Autonomous System's topology 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 N. Router interface names are prefixed by 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. 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 ______________________________________ (Figure not included in text version.) Figure 1: Network map components ______________________________________
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 ______________________________________ (Figure not included in text version.) Figure 2: A sample Autonomous System ______________________________________ __________________________________________ (Figures not included in text version.) Figure 3: The resulting directed graph Figure 4: Individual link state components __________________________________________
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. 2.1 The shortest-path tree When no OSPF areas are configured, each router in the Autonomous System has an identical topological 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 route 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 serial line (in this case, the serial line between routers RT6 and RT10). 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. ______________________________________ (Figure not included in text version.) Figure 5: The SPF tree for router RT6 ______________________________________
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. 2.2 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 EGP, 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 equivalent to the link state metric. Type 2 external metrics are 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. Here is 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 external route, the distance from Router RT6 is calculated as the sum of the external route's cost and the distance from
Router RT6 to the advertising router. For every external destination, the router advertising the shortest route is discovered, and the next hop to the advertising router becomes the next hop to the destination. 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 is 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 EGP 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 external advertisements. 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 external advertisements. In each external advertisement, router RT6 would specify the correct Autonomous System exit point to use for the destination through appropriate setting of the advertisement's "forwarding address" field. 2.3 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. 2.4 TOS-based routing OSPF can calculate a separate set of routes for each IP Type of Service. The IP TOS values are represented in OSPF exactly as they appear in the IP packet header. This means that, for any destination, there can potentially be multiple routing table entries, one for each IP TOS. Up to this point, all examples shown have assumed that routes do not vary on TOS. In order to differentiate routes based on TOS, separate interface costs can be configured for each TOS. For example, in Figure 2 there could be multiple costs (one for each TOS) listed for each interface. A cost for TOS 0 must always be specified. When interface costs vary based on TOS, a separate shortest path tree is calculated for each TOS (see Section 2.1). In addition, external costs can vary based on TOS. For example, in Figure 2 router RT7 could advertise a separate type 1 external metric for each TOS. Then, when calculating the TOS X distance to network N15 the cost of the shortest TOS X path to RT7 would be added to the TOS X cost advertised by RT7
(see Section 2.2). All OSPF implementations must be capable of calculating routes based on TOS. However, OSPF routers can be configured to route all packets on the TOS 0 path (see Appendix C), eliminating the need to calculate non- zero TOS paths. This can be used to conserve routing table space and processing resources in the router. These TOS-0-only routers can be mixed with routers that do route based on TOS. TOS-0-only routers will be avoided as much as possible when forwarding traffic requesting a non-zero TOS. It may be the case that no path exists for some non-zero TOS, even if the router is calculating non-zero TOS paths. In that case, packets requesting that non-zero TOS are routed along the TOS 0 path (see Section 11.1). 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 SPF routing algorithm. This means that each area has its own topological 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 SPF domain. With the introduction of areas, it is no longer true that all routers in the AS have an identical topological database. A router actually has a separate topological 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 topological 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 routing 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 backbone consists of those networks not contained in any area, their attached routers, and those routers that belong to multiple areas. The backbone must be contiguous. It is possible to define areas in such a way that the backbone is no longer contiguous. In this case the system administrator must restore backbone connectivity by configuring 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 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. The backbone is responsible for distributing routing information between areas. The backbone itself has all of the properties of an area. The topology of the backbone is invisible to each of the areas, while the backbone itself knows nothing of the topology of the areas. 3.2 Inter-area routing When routing a packet between two 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 and each of the areas as spokes. 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 other networks 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 directly connected networks belonging to the same area. Routers with only backbone interfaces also belong to this category. These routers run a single copy of the basic routing algorithm. Area border routers A router that attaches to multiple areas. Area border routers run multiple copies of the basic algorithm, one copy for each attached area and an additional copy for the backbone. Area border routers condense the topological information of their attached areas for distribution to the backbone. The backbone in turn distributes the information to the other areas. Backbone routers A router that has an interface to the backbone. This includes all routers that interface to more than one area (i.e., area border routers). However, backbone routers do not have to be area border routers. Routers with all interfaces connected to the backbone are considered to be internal routers. AS boundary routers A router that exchanges routing information with routers belonging to other Autonomous Systems. Such a router has AS external routes that are advertised throughout the Autonomous System. The path to each AS boundary router is known by every router in the AS. This classification is completely independent of the previous classifications: AS boundary routers may be internal or area border routers, and may or may not participate in the backbone. 3.4 A sample area configuration Figure 6 shows a sample area configuration. The first area consists of networks N1-N4, along with their attached routers RT1-RT4. The second area consists of networks N6-N8, along with their attached routers RT7,
RT8, RT10, RT11. The third area consists of networks N9-N11 and host H1, along with their attached routers RT9, RT11, RT12. The third area has been configured so that networks N9-N11 and host H1 will all be grouped into a single route, when advertised external to the area (see Section 3.5 for more details). In Figure 6, routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are internal routers. Routers RT3, RT4, RT7, RT10 and RT11 are area border routers. Finally as before, routers RT5 and RT7 are AS boundary routers. Figure 7 shows the resulting topological database for the Area 1. The figure completely describes that area's intra-area routing. It also shows the complete view of the internet for the two internal routers RT1 and RT2. It is the job of the area border routers, RT3 and RT4, to advertise into Area 1 the distances to all destinations external to the area. These are indicated in Figure 7 by the dashed stub routes. Also, RT3 and RT4 must advertise into Area 1 the location of the AS boundary routers RT5 and RT7. Finally, external advertisements from RT5 and RT7 are flooded throughout the entire AS, and in particular throughout Area 1. These advertisements are included in Area 1's database, and yield routes to networks N12-N15. Routers RT3 and RT4 must also summarize Area 1's topology for distribution to the backbone. Their backbone advertisements are shown in Table 4. These summaries show which networks are contained in Area 1 (i.e., networks N1-N4), and the distance to these networks from the routers RT3 and RT4 respectively. The topological database for the backbone is shown in Figure 8. The set of routers pictured are the backbone routers. Router RT11 is a backbone router because it belongs to two areas. In order to make the backbone connected, a virtual link has been configured between routers R10 and R11. __________________________________________ (Figure not included in text version.) Figure 6: A sample OSPF area configuration __________________________________________
Network RT3 adv. RT4 adv. _____________________________ N1 4 4 N2 4 4 N3 1 1 N4 2 3 Table 4: Networks advertised to the backbone by routers RT3 and RT4. ______________________________________ (Figure not included in text version.) Figure 7: Area 1's Database Figure 8: The backbone database ______________________________________ Again, routers RT3, RT4, RT7, RT10 and RT11 are area border routers. As routers RT3 and RT4 did above, they have condensed the routing information of their attached areas for distribution via the backbone; these are the dashed stubs that appear in Figure 8. Remember that the third area has been configured to condense networks N9-N11 and Host H1 into a single route. This yields a single dashed line for networks N9- N11 and Host H1 in Figure 8. Routers RT5 and RT7 are AS boundary routers; their externally derived information also appears on the graph in Figure 8 as stubs. The backbone enables the exchange of summary information between area border routers. Every area border router hears the area summaries from all other area border routers. It then forms a picture of the distance to all networks outside of its area by examining the collected advertisements, and adding in the backbone distance to each advertising router. Again using routers RT3 and RT4 as an example, the procedure goes as follows: They first calculate the SPF tree for the backbone. This gives the distances to all other area border routers. Also noted are the distances to networks (Ia and Ib) and AS boundary routers (RT5 and RT7) that belong to the backbone. This calculation is shown in Table 5. Next, by looking at the area summaries from these area border routers, RT3 and RT4 can determine the distance to all networks outside their
Area border dist from dist from router RT3 RT4 ______________________________________ to RT3 * 21 to RT4 22 * to RT7 20 14 to RT10 15 22 to RT11 18 25 ______________________________________ to Ia 20 27 to Ib 15 22 ______________________________________ to RT5 14 8 to RT7 20 14 Table 5: Backbone distances calculated by routers RT3 and RT4. area. These distances are then advertised internally to the area by RT3 and RT4. The advertisements that router RT3 and RT4 will make into Area 1 are shown in Table 6. Note that Table 6 assumes that an area range has been configured for the backbone which groups I5 and I6 into a single advertisement. The information imported into Area 1 by routers RT3 and RT4 enables an internal router, such as RT1, to choose an area border router intelligently. Router RT1 would use RT4 for traffic to network N6, RT3 for traffic to network N10, and would load share between the two for Destination RT3 adv. RT4 adv. _________________________________ Ia,Ib 15 22 N6 16 15 N7 20 19 N8 18 18 N9-N11,H1 19 26 _________________________________ RT5 14 8 RT7 20 14 Table 6: Destinations advertised into Area 1 by routers RT3 and RT4.
traffic to network N8. Router RT1 can also determine in this manner the shortest path to the AS boundary routers RT5 and RT7. Then, by looking at RT5 and RT7's external advertisements, router RT1 can decide between RT5 or RT7 when sending to a destination in another Autonomous System (one of the networks N12-N15). Note that a failure of the line between routers RT6 and RT10 will cause the backbone to become disconnected. Configuring another virtual link between routers RT7 and RT10 will give the backbone more connectivity and more resistance to such failures. Also, a virtual link between RT7 and RT10 would allow a much shorter path between the third area (containing N9) and the router RT7, which is advertising a good route to external network N12. 3.5 IP subnetting support OSPF attaches an IP address mask to each advertised route. The mask indicates the range of addresses being described by the particular route. For example, a summary advertisement for the destination 18.104.22.168 with a mask of 0xffff0000 actually is describing a single route to the collection of destinations 22.214.171.124 - 126.96.36.199. Similarly, host routes are always advertised with a mask of 0xffffffff, indicating the presence of only a single destination. Including the mask with each advertised destination enables the implementation of what is commonly referred to as variable-length subnet masks. This means that a single IP class A, B, or C network number can be broken up into many subnets of various sizes. For example, the network 188.8.131.52 could be broken up into 64 variable-sized subnets: 16 subnets of size 4K, 16 subnets of size 256, and 32 subnets of size 8. Table 7 shows some of the resulting network addresses together with their masks: Network address IP address mask Subnet size _______________________________________________ 184.108.40.206 0xfffff000 4K 220.127.116.11 0xffffff00 256 18.104.22.168 0xfffffff8 8 Table 7: Some sample subnet sizes.
There are many possible ways of dividing up a class A, B, and C network into variable sized subnets. The precise procedure for doing so is beyond the scope of this specification. The specification however establishes the following guideline: When an IP packet is forwarded, it is always forwarded to the network that is the best match for the packet's destination. Here best match is synonymous with the longest or most specific match. For example, the default route with destination of 0.0.0.0 and mask 0x00000000 is always a match for every IP destination. Yet it is always less specific than any other match. Subnet masks must be assigned so that the best match for any IP destination is unambiguous. The OSPF area concept is modelled after an IP subnetted network. OSPF areas have been loosely defined to be a collection of networks. In actuality, an OSPF area is specified to be a list of address ranges (see Section C.2 for more details). Each address range is defined as an [address,mask] pair. Many separate networks may then be contained in a single address range, just as a subnetted network is composed of many separate subnets. Area border routers then summarize the area contents (for distribution to the backbone) by advertising a single route for each address range. The cost of the route is the minimum cost to any of the networks falling in the specified range. For example, an IP subnetted network can be configured as a single OSPF area. In that case, the area would be defined as a single address range: a class A, B, or C network number along with its natural IP mask. Inside the area, any number of variable sized subnets could be defined. External to the area, a single route for the entire subnetted network would be distributed, hiding even the fact that the network is subnetted at all. The cost of this route is the minimum of the set of costs to the component subnets. 3.6 Supporting stub areas In some Autonomous Systems, the majority of the topological database may consist of external advertisements. An OSPF external advertisement is usually flooded throughout the entire AS. However, OSPF allows certain areas to be configured as "stub areas". External advertisements are not flooded into/throughout stub areas; routing to AS external destinations in these areas is based on a (per-area) default only. This reduces the topological database size, and therefore the memory requirements, for a stub area's internal routers. In order to take advantage of the OSPF stub area support, default routing must be used in the stub area. This is accomplished as follows. One or more of the stub area's area border routers must advertise a default route into the stub area via summary advertisements. These
summary defaults are flooded throughout the stub area, but no further. (For this reason these defaults pertain only to the particular stub area). These summary default routes will match any destination that is not explicitly reachable by an intra-area or inter-area path (i.e., AS external destinations). An area can be configured as stub when there is a single exit point from the area, or when the choice of exit point need not be made on a per- external-destination basis. For example, area 3 in Figure 6 could be configured as a stub area, because all external traffic must travel though its single area border router RT11. If area 3 were configured as a stub, router RT11 would advertise a default route for distribution inside area 3 (in a summary advertisement), instead of flooding the external advertisements for networks N12-N15 into/throughout the area. The OSPF protocol ensures that all routers belonging to an area agree on whether the area has been configured as a stub. This guarantees that no confusion will arise in the flooding of external advertisements. There are a couple of restrictions on the use of stub areas. Virtual links cannot be configured through stub areas. In addition, AS boundary routers cannot be placed internal to stub areas. 3.7 Partitions of areas OSPF does not actively attempt to repair area partitions. When an area becomes partitioned, each component simply becomes a separate area. The backbone then performs routing between the new areas. Some destinations reachable via intra-area routing before the partition will now require inter-area routing. In the previous section, an area was described as a list of address ranges. Any particular address range must still be completely contained in a single component of the area partition. This has to do with the way the area contents are summarized to the backbone. Also, the backbone itself must not partition. If it does, parts of the Autonomous System will become unreachable. Backbone partitions can be repaired by configuring virtual links (see Section 15). Another way to think about area partitions is to look at the Autonomous System graph that was introduced in Section 2. Area IDs can be viewed as colors for the graph's edges. Each edge of the graph connects to a network, or is itself a point-to-point network. In either case, the edge is colored with the network's Area ID. A group of edges, all having the same color, and interconnected by vertices, represents an area. If the topology of the Autonomous System
is intact, the graph will have several regions of color, each color being a distinct Area ID. When the AS topology changes, one of the areas may become partitioned. The graph of the AS will then have multiple regions of the same color (Area ID). The routing in the Autonomous System will continue to function as long as these regions of same color are connected by the single backbone region.