RFC 2178 OSPF Version 2 July 1997 2.1.1 Representation of non-broadcast networks .............. 13 2.1.2 An example link-state database ........................ 14 2.2 The shortest-path tree ................................ 18 2.3 Use of external routing information ................... 20 2.4 Equal-cost multipath .................................. 22 3 Splitting the AS into Areas ........................... 22 3.1 The backbone of the Autonomous System ................. 23 3.2 Inter-area routing .................................... 23 3.3 Classification of routers ............................. 24 3.4 A sample area configuration ........................... 25 3.5 IP subnetting support ................................. 31 3.6 Supporting stub areas ................................. 32 3.7 Partitions of areas ................................... 33 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 .............................. 40 6 The Area Data Structure ............................... 42 7 Bringing Up Adjacencies ............................... 44 7.1 The Hello Protocol .................................... 44 7.2 The Synchronization of Databases ...................... 45 7.3 The Designated Router ................................. 46 7.4 The Backup Designated Router .......................... 47 7.5 The graph of adjacencies .............................. 48 8 Protocol Packet Processing ............................ 49 8.1 Sending protocol packets .............................. 49 8.2 Receiving protocol packets ............................ 51 9 The Interface Data Structure .......................... 54 9.1 Interface states ...................................... 57 9.2 Events causing interface state changes ................ 59 9.3 The Interface state machine ........................... 61 9.4 Electing the Designated Router ........................ 64 9.5 Sending Hello packets ................................. 66 9.5.1 Sending Hello packets on NBMA networks ................ 67 10 The Neighbor Data Structure ........................... 68 10.1 Neighbor states ....................................... 70 10.2 Events causing neighbor state changes ................. 75 10.3 The Neighbor state machine ............................ 76 10.4 Whether tocome adjacent ............................ 82 10.5 Receiving Hello Packets ............................... 83 10.6 Receiving Database Description Packets ................ 85 10.7 Receiving Link State Request Packets .................. 88 10.8 Sending Database Description Packets .................. 89 10.9 Sending Link State Request Packets .................... 90 10.10 An Example ............................................ 91 Moy Standards Track