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