rfc2328.txt
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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, ZhaohuiMoy Standards Track [Page 12] RFC 2328 OSPF Version 2 April 1998 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.2. The Link-state Database: organization and calculations The following subsections describe the organization of OSPF's link- state database, and the routing calculations that are performed on the database in order to produce a router's routing table. 2.1. Representation of routers and networks The Autonomous System's link-state 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. Networks can be either transit or stub networks. Transit networks are those capable of carrying data traffic that is neither locally originated nor locally destined. A transit network is represented by a graph vertex having both incoming and outgoing edges. A stub network's vertex has only incoming edges. The neighborhood of each network node in the graph depends on the network's type (point-to-point, broadcast, NBMA or Point- to-MultiPoint) and the number of routers having an interface to the network. Three cases are depicted in Figure 1a. Rectangles indicate routers. Circles and oblongs indicate 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 networks with their connected routers, with the resulting graphs shown on the right.Moy Standards Track [Page 13] RFC 2328 OSPF Version 2 April 1998 **FROM** * |RT1|RT2| +---+Ia +---+ * ------------ |RT1|------|RT2| T RT1| | X | +---+ Ib+---+ O RT2| X | | * Ia| | X | * Ib| X | | Physical point-to-point networks **FROM** +---+ * |RT7| * |RT7| N3| +---+ T ------------ | O RT7| | | +----------------------+ * N3| X | | N3 * Stub 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 | | +---+ +---+ Broadcast or NBMA networks Figure 1a: Network map componentsMoy Standards Track [Page 14] RFC 2328 OSPF Version 2 April 1998 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. The top of Figure 1a shows two routers connected by a point-to- point link. In the resulting link-state database graph, the two router vertices are directly connected by a pair of edges, one in each direction. Interfaces to point-to-point networks need not be assigned IP addresses. When interface addresses are assigned, they are modelled as stub links, with each router advertising a stub connection to the other router's interface address. Optionally, an IP subnet can be assigned to the point- to-point network. In this case, both routers advertise a stub link to the IP subnet, instead of advertising each others' IP interface addresses. The middle of Figure 1a shows a network with only one attached router (i.e., a stub network). In this case, the network appears on the end of a stub connection in the link-state database's graph. When multiple routers are attached to a broadcast network, the link-state database graph shows all routers bidirectionally connected to the network vertex. This is pictured at the bottom of Figure 1a. 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. 2.1.1. Representation of non-broadcast networks As mentioned previously, OSPF can run over non-broadcast networks in one of two modes: NBMA or Point-to-MultiPoint. The choice of mode determines the way that the HelloMoy Standards Track [Page 15] RFC 2328 OSPF Version 2 April 1998 protocol and flooding work over the non-broadcast network, and the way that the network is represented in the link- state database. In NBMA mode, OSPF emulates operation over a broadcast network: a Designated Router is elected for the NBMA network, and the Designated Router originates an LSA for the network. The graph representation for broadcast networks and NBMA networks is identical. This representation is pictured in the middle of Figure 1a. NBMA mode is the most efficient way to run OSPF over non- broadcast networks, both in terms of link-state database size and in terms of the amount of routing protocol traffic. However, it has one significant restriction: it requires all routers attached to the NBMA network to be able to communicate directly. This restriction may be met on some non-broadcast networks, such as an ATM subnet utilizing SVCs. But it is often not met on other non-broadcast networks, such as PVC-only Frame Relay networks. On non- broadcast networks where not all routers can communicate directly you can break the non-broadcast network into logical subnets, with the routers on each subnet being able to communicate directly, and then run each separate subnet as an NBMA network (see [Ref15]). This however requires quite a bit of administrative overhead, and is prone to misconfiguration. It is probably better to run such a non- broadcast network in Point-to-Multipoint mode. In Point-to-MultiPoint mode, OSPF treats all router-to- router connections over the non-broadcast network as if they were point-to-point links. No Designated Router is elected for the network, nor is there an LSA generated for the network. In fact, a vertex for the Point-to-MultiPoint network does not appear in the graph of the link-state database. Figure 1b illustrates the link-state database representation of a Point-to-MultiPoint network. On the left side of the figure, a Point-to-MultiPoint network is pictured. It is assumed that all routers can communicate directly, except for routers RT4 and RT5. I3 though I6 indicate the routers'Moy Standards Track [Page 16] RFC 2328 OSPF Version 2 April 1998 IP interface addresses on the Point-to-MultiPoint network. In the graphical representation of the link-state database, routers that can communicate directly over the Point-to- MultiPoint network are joined by bidirectional edges, and each router also has a stub connection to its own IP interface address (which is in contrast to the representation of real point-to-point links; see Figure 1a). On some non-broadcast networks, use of Point-to-MultiPoint mode and data-link protocols such as Inverse ARP (see [Ref14]) will allow autodiscovery of OSPF neighbors even though broadcast support is not available. **FROM** +---+ +---+ |RT3| |RT4| |RT3|RT4|RT5|RT6| +---+ +---+ * -------------------- I3| N2 |I4 * RT3| | X | X | X | +----------------------+ T RT4| X | | | X |⌨️ 快捷键说明
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