Network Working Group J. Moy Request for Comments: 1583 Proteon, Inc. Obsoletes: 1247 March 1994 Category: Standards Track OSPF Version 2 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) 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 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. OSPF Version 2 was originally documented in RFC 1247. The differences between RFC 1247 and this memo are explained in Appendix E. The differences consist of bug fixes and clarifications, and are backward-compatible in nature. Implementations of RFC 1247 and of this memo will interoperate. Please send comments to ospf@gated.cornell.edu. Moy [Page 1] RFC 1583 OSPF Version 2 March 1994 Table of Contents 1 Introduction ........................................... 5 1.1 Protocol Overview ...................................... 5 1.2 Definitions of commonly used terms ..................... 6 1.3 Brief history of link-state routing technology ......... 9 1.4 Organization of this document .......................... 9 2 The Topological Database .............................. 10 2.1 The shortest-path tree ................................ 13 2.2 Use of external routing information ................... 16 2.3 Equal-cost multipath .................................. 20 2.4 TOS-based routing ..................................... 20 3 Splitting the AS into Areas ........................... 21 3.1 The backbone of the Autonomous System ................. 22 3.2 Inter-area routing .................................... 22 3.3 Classification of routers ............................. 23 3.4 A sample area configuration ........................... 24 3.5 IP subnetting support ................................. 30 3.6 Supporting stub areas ................................. 31 3.7 Partitions of areas ................................... 32 4 Functional Summary .................................... 34 4.1 Inter-area routing .................................... 35 4.2 AS external routes .................................... 35 4.3 Routing protocol packets .............................. 35 4.4 Basic implementation requirements ..................... 38 4.5 Optional OSPF capabilities ............................ 39 5 Protocol data structures .............................. 41 6 The Area Data Structure ............................... 42 7 Bringing Up Adjacencies ............................... 45 7.1 The Hello Protocol .................................... 45 7.2 The Synchronization of Databases ...................... 46 7.3 The Designated Router ................................. 47 7.4 The Backup Designated Router .......................... 48 7.5 The graph of adjacencies .............................. 49 8 Protocol Packet Processing ............................ 50 8.1 Sending protocol packets .............................. 51 8.2 Receiving protocol packets ............................ 53 9 The Interface Data Structure .......................... 55 9.1 Interface states ...................................... 58 9.2 Events causing interface state changes ................ 61 9.3 The Interface state machine ........................... 62 9.4 Electing the Designated Router ........................ 65 9.5 Sending Hello packets ................................. 67 9.5.1 Sending Hello packets on non-broadcast networks ....... 68 10 The Neighbor Data Structure ........................... 69 10.1 Neighbor states ....................................... 72 10.2 Events causing neighbor state changes ................. 75 10.3 The Neighbor state machine ............................ 77 Moy [Page 2] RFC 1583 OSPF Version 2 March 1994 10.4 Whether to become adjacent ............................ 83 10.5 Receiving Hello Packets ............................... 83 10.6 Receiving Database Description Packets ................ 86 10.7 Receiving Link State Request Packets .................. 89 10.8 Sending Database Description Packets .................. 89 10.9 Sending Link State Request Packets .................... 90 10.10 An Example ............................................ 91 11 The Routing Table Structure ........................... 93 11.1 Routing table lookup .................................. 96 11.2 Sample routing table, without areas ................... 97 11.3 Sample routing table, with areas ...................... 98 12 Link State Advertisements ............................ 100 12.1 The Link State Advertisement Header .................. 101 12.1.1 LS age ............................................... 102 12.1.2 Options .............................................. 102 12.1.3 LS type .............................................. 103 12.1.4 Link State ID ........................................ 103 12.1.5 Advertising Router ................................... 105 12.1.6 LS sequence number ................................... 105 12.1.7 LS checksum .......................................... 106 12.2 The link state database .............................. 107 12.3 Representation of TOS ................................ 108 12.4 Originating link state advertisements ................ 109 12.4.1 Router links ......................................... 112 12.4.2 Network links ........................................ 118 12.4.3 Summary links ........................................ 120 12.4.4 Originating summary links into stub areas ............ 123 12.4.5 AS external links .................................... 124 13 The Flooding Procedure ............................... 126 13.1 Determining which link state is newer ................ 130 13.2 Installing link state advertisements in the database . 130 13.3 Next step in the flooding procedure .................. 131 13.4 Receiving self-originated link state ................. 134 13.5 Sending Link State Acknowledgment packets ............ 135 13.6 Retransmitting link state advertisements ............. 136 13.7 Receiving link state acknowledgments ................. 138 14 Aging The Link State Database ........................ 139 14.1 Premature aging of advertisements .................... 139 15 Virtual Links ........................................ 140 16 Calculation Of The Routing Table ..................... 142 16.1 Calculating the shortest-path tree for an area ....... 143 16.1.1 The next hop calculation ............................. 149 16.2 Calculating the inter-area routes .................... 150 16.3 Examining transit areas' summary links ............... 152 16.4 Calculating AS external routes ....................... 154 16.5 Incremental updates -- summary link advertisements ... 156 16.6 Incremental updates -- AS external link advertisements 157 16.7 Events generated as a result of routing table changes 157 Moy [Page 3] RFC 1583 OSPF Version 2 March 1994 16.8 Equal-cost multipath ................................. 158 16.9 Building the non-zero-TOS portion of the routing table 158 Footnotes ............................................ 161 References ........................................... 164 A OSPF data formats .................................... 166 A.1 Encapsulation of OSPF packets ........................ 166 A.2 The Options field .................................... 168 A.3 OSPF Packet Formats .................................. 170 A.3.1 The OSPF packet header ............................... 171 A.3.2 The Hello packet ..................................... 173 A.3.3 The Database Description packet ...................... 175 A.3.4 The Link State Request packet ........................ 177 A.3.5 The Link State Update packet ......................... 179 A.3.6 The Link State Acknowledgment packet ................. 181 A.4 Link state advertisement formats ..................... 183 A.4.1 The Link State Advertisement header .................. 184 A.4.2 Router links advertisements .......................... 186 A.4.3 Network links advertisements ......................... 190 A.4.4 Summary link advertisements .......................... 192 A.4.5 AS external link advertisements ...................... 194 B Architectural Constants .............................. 196 C Configurable Constants ............................... 198 C.1 Global parameters .................................... 198 C.2 Area parameters ...................................... 198 C.3 Router interface parameters .......................... 200 C.4 Virtual link parameters .............................. 202 C.5 Non-broadcast, multi-access network parameters ....... 203 C.6 Host route parameters ................................ 203 D Authentication ....................................... 205 D.1 AuType 0 -- No authentication ........................ 205 D.2 AuType 1 -- Simple password .......................... 205 E Differences from RFC 1247 ............................ 207 E.1 A fix for a problem with OSPF Virtual links .......... 207 E.2 Supporting supernetting and subnet 0 ................. 208 E.3 Obsoleting LSInfinity in router links advertisements . 209 E.4 TOS encoding updated ................................. 209 E.5 Summarizing routes into transit areas ................ 210 E.6 Summarizing routes into stub areas ................... 210 E.7 Flushing anomalous network links advertisements ...... 210 E.8 Required Statistics appendix deleted ................. 211 E.9 Other changes ........................................ 211 F. An algorithm for assigning Link State IDs ............ 213 Security Considerations .............................. 216 Author's Address ..................................... 216 Moy [Page 4] RFC 1583 OSPF Version 2 March 1994 1. Introduction This document is a specification of the Open Shortest Path First (OSPF) TCP/IP internet routing protocol. OSPF is classified as an Interior Gateway Protocol (IGP). This means that it distributes routing information between routers belonging to a single Autonomous System. The OSPF protocol is based on link-state or SPF technology. This is a departure from the Bellman-Ford base used by traditional TCP/IP 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 TCP/IP 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 Fred Baker, Jeffrey Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra Jujjavarapu, Milo Medin, Kannan Varadhan 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 a link-state 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 Moy [Page 5] RFC 1583 OSPF Version 2 March 1994 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 subnetting. 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 This section provides definitions for terms that have a specific meaning to the OSPF 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. Moy [Page 6] RFC 1583 OSPF Version 2 March 1994 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. Interior Gateway Protocol The routing protocol spoken by the routers belonging to an Autonomous system. Abbreviated as IGP. Each Autonomous System has a single IGP. Separate 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 memo, an IP network/subnet/supernet. 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/supernet. 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 Moy [Page 7] RFC 1583 OSPF Version 2 March 1994 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 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 X.25 PDNs, and the ethernet data link layer for ethernets. Moy [Page 8] RFC 1583 OSPF Version 2 March 1994 1.3. Brief history of link-state routing technology OSPF is a link state routing protocol. Such protocols are also referred to in the literature as SPF-based or distributed- database protocols. This section gives a brief description of the developments in link-state technology that have influenced the OSPF protocol. The first link-state 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 link-state 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 a link-state protocol. This was accomplished by introducing mechanisms which enabled the interval between link state advertisement originations to be increased by an order of magnitude. A link-state 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 TCP/IP internets. 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 Moy [Page 9] RFC 1583 OSPF Version 2 March 1994 4-16 explain the protocol's mechanisms in detail. Packet formats, protocol constants and configuration items 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 memo. 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. Moy [Page 10] RFC 1583 OSPF Version 2 March 1994 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 Moy [Page 11] RFC 1583 OSPF Version 2 March 1994 **FROM** * |RT1|RT2| +---+Ia +---+ * ------------ |RT1|------|RT2| T RT1| | X | +---+ Ib+---+ O RT2| X | | * Ia| | X | * Ib| X | | Physical point-to-point networks **FROM** +---+ +---+ |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 **FROM** +---+ * |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. Moy [Page 12] RFC 1583 OSPF Version 2 March 1994 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 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. 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 Moy [Page 13] RFC 1583 OSPF Version 2 March 1994 + | 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+---+ | + |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 System Moy [Page 14] RFC 1583 OSPF Version 2 March 1994 **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. Moy [Page 15] RFC 1583 OSPF Version 2 March 1994 **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 links N9's network links advertisement advertisement 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. 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. 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 Moy [Page 16] RFC 1583 OSPF Version 2 March 1994 RT6(origin) RT5 o------------o-----------o Ib /|\ 6 |\ 7 8/8|8\ | \ / | \ | \ 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 2.2 Moy [Page 17] RFC 1583 OSPF Version 2 March 1994 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. 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. 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 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. Moy [Page 18] RFC 1583 OSPF Version 2 March 1994 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 a