RFC 1583

Network Working Group
Request for Comments: 1583
Obsoletes: 1247
Category: Standards Track

J. Moy
Proteon, Inc.
March 1994

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.

Table of Contents

• 1. Introduction
  • 1.1. Protocol overview
  • 1.2. Definitions of commonly used terms
  • 1.3. Brief history of link-state routing technology
  • 1.4. Organization of this document
• 2. The Topological Database
  • 2.1. The shortest-path tree
  • 2.2. Use of external routing information
  • 2.3. Equal-cost multipath
  • 2.4. TOS-based routing
• 3. Splitting the AS into Areas
  • 3.1. The backbone of the Autonomous System
  • 3.2. Inter-area routing
  • 3.3. Classification of routers
  • 3.4. A sample area configuration
  • 3.5. IP subnetting support
  • 3.6. Supporting stub areas
  • 3.7. Partitions of areas
• 4. Functional Summary
  • 4.1. Inter-area routing
  • 4.2. AS external routes
  • 4.3. Routing protocol packets
  • 4.4. Basic implementation requirements
  • 4.5. Optional OSPF capabilities
• 5. Protocol Data Structures
• 6. The Area Data Structure
• 7. Bringing Up Adjacencies
  • 7.1. The Hello Protocol
  • 7.2. The Synchronization of Databases
  • 7.3. The Designated Router
  • 7.4. The Backup Designated Router
  • 7.5. The graph of adjacencies
• 8. Protocol Packet Processing
  • 8.1. Sending protocol packets
  • 8.2. Receiving protocol packets
• 9. The Interface Data Structure
  • 9.1. Interface states
  • 9.2. Events causing interface state changes
  • 9.3. The Interface state machine
  • 9.4. Electing the Designated Router
  • 9.5. Sending Hello packets
    • 9.5.1. Sending Hello packets on non-broadcast networks
• 10. The Neighbor Data Structure
  • 10.1. Neighbor states
  • 10.2. Events causing neighbor state changes
  • 10.3. The Neighbor state machine
  • 10.4. Whether to become adjacent
  • 10.5. Receiving Hello Packets
  • 10.6. Receiving Database Description Packets
  • 10.7. Receiving Link State Request Packets
  • 10.8. Sending Database Description Packets
  • 10.9. Sending Link State Request Packets
  • 10.10. An Example
• 11. The Routing Table Structure
  • 11.1. Routing table lookup
  • 11.2. Sample routing table, without areas
  • 11.3. Sample routing table, with areas
• 12. Link State Advertisements
  • 12.1. The Link State Advertisement Header
    • 12.1.1. LS age
    • 12.1.2. Options
    • 12.1.3. LS type
    • 12.1.4. Link State ID
    • 12.1.5. Advertising Router
    • 12.1.6. LS sequence number
    • 12.1.7. LS checksum
  • 12.2. The link state database
  • 12.3. Representation of TOS
  • 12.4. Originating link state advertisements
    • 12.4.1. Router links
    • 12.4.2. Network links
    • 12.4.3. Summary links
    • 12.4.4. Originating summary links into stub areas
    • 12.4.5. AS external links
• 13. The Flooding Procedure
  • 13.1. Determining which link state is newer
  • 13.2. Installing link state advertisements in the database
  • 13.3. Next step in the flooding procedure
  • 13.4. Receiving self-originated link state
  • 13.5. Sending Link State Acknowledgment packets
  • 13.6. Retransmitting link state advertisements
  • 13.7. Receiving link state acknowledgments
• 14. Aging The Link State Database
  • 14.1. Premature aging of advertisements
• 15. Virtual Links
• 16. Calculation Of The Routing Table
  • 16.1. Calculating the shortest-path tree for an area
    • 16.1.1. The next hop calculation
  • 16.2. Calculating the inter-area routes
  • 16.3. Examining transit areas' summary links
  • 16.4. Calculating AS external routes
  • 16.5. Incremental updates -- summary link advertisements
  • 16.6. Incremental updates -- AS external link advertisements
  • 16.7. Events generated as a result of routing table changes
  • 16.8. Equal-cost multipath
  • 16.9. Building the non-zero-TOS portion of the routing table
• Footnotes
• References
• A. OSPF data formats
  • A.1 Encapsulation of OSPF packets
  • A.2 The Options field
  • A.3 OSPF Packet Formats
    • A.3.1 The OSPF packet header
    • A.3.2 The Hello packet
    • A.3.3 The Database Description packet
    • A.3.4 The Link State Request packet
    • A.3.5 The Link State Update packet
    • A.3.6 The Link State Acknowledgment packet
  • A.4 Link state advertisement formats
    • A.4.1 The Link State Advertisement header
    • A.4.2 Router links advertisements
    • A.4.3 Network links advertisements
    • A.4.4 Summary link advertisements
    • A.4.5 AS external link advertisements
• B. Architectural Constants
• C. Configurable Constants
  • C.1 Global parameters
  • C.2 Area parameters
  • C.3 Router interface parameters
  • C.4 Virtual link parameters
  • C.5 Non-broadcast, multi-access network parameters
  • C.6 Host route parameters
• D. Authentication
• E. Differences from RFC 1247
  • E.1 A fix for a problem with OSPF Virtual links
  • E.2 Supporting supernetting and subnet 0
  • E.3 Obsoleting LSInfinity in router links advertisements
  • E.4 TOS encoding updated
  • E.5 Summarizing routes into transit areas
  • E.6 Summarizing routes into stub areas
  • E.7 Flushing anomalous network links advertisements
  • E.8 Required Statistics appendix deleted
  • E.9 Other changes
• F. An algorithm for assigning Link State IDs
• Security Considerations
• Author's Address

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