1. Introduction

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 CIDR 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.

   1.1. Protocol overview

   OSPF routes IP packets based solely on the destination IP address 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. This database is referred to as the link-state database. 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 link-state 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.

   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; in fact, separate authentication schemes can be configured for each IP subnet.

   Externally derived routing data (e.g., routes learned from an Exterior Gateway Protocol such as BGP; see [Ref23]) is advertised 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 boundary 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 [Ref13] 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.

   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.

   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 OSPF protocol makes further use of multicast capabilities, if they exist. Each pair of routers on a broadcast network is assumed to be able to communicate directly. 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 maintained on these nets using OSPF's Hello Protocol. However, due to the lack of broadcast capability, some configuration information may be necessary to aid in the discovery of neighbors. On non-broadcast 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.

   OSPF runs in one of two modes over non-broadcast networks. The first mode, called non-broadcast multi-access or NBMA, simulates the operation of OSPF on a broadcast network. The second mode, called Point-to-MultiPoint, treats the non- broadcast network as a collection of point-to-point links. Non-broadcast networks are referred to as NBMA networks or Point-to-MultiPoint networks, depending on OSPF's mode of operation over the network.

   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. Neighbor relationships are maintained by, and usually 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 Unit of data describing the local state of a router or network. For a router, 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 link state database. Throughout this memo, link state advertisement is abbreviated as LSA.

   Hello Protocol The part of the OSPF protocol used to establish and maintain neighbor relationships. On broadcast networks the Hello Protocol can also dynamically discover neighboring routers.

   Flooding The part of the OSPF protocol that distributes and synchronizes the link-state database between OSPF routers.

   Designated Router Each broadcast and NBMA network that has at least two attached routers has a Designated Router. The Designated

   Router generates an LSA for the 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 broadcast or NBMA network. This in turn reduces the amount of routing protocol traffic and the size of the link-state 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.

   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 [Ref3]. 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 [Ref4]. These modifications dealt with increasing the fault tolerance of the routing protocol through, among other things, adding a checksum to the LSAs (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 LSA 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 [Ref2]. 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 an LSA for the network.

   The OSPF Working Group 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 algorithms have been tailored 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 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. Architectural constants are summarized in Appendix B. Configurable constants are summarized 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.

   1.5. Acknowledgments

   The author would like to thank Ran Atkinson, Fred Baker, Jeffrey Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra Jujjavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui

   Zhang and the rest of the OSPF Working Group for the ideas and support they have given to this project.

   The OSPF Point-to-MultiPoint interface is based on work done by Fred Baker.

   The OSPF Cryptographic Authentication option was developed by Fred Baker and Ran Atkinson.