Sam Halabi
Network Consulting Engineer
The Open Shortest Path First Protocol (OSPF), defined in RFC 1583, is an Interior Gateway Protocol used to distribute routing information within a single Autonomous System. This paper examines how OSPF works and how it can be used to design and build today's large and complicated networks.
I.0 Introduction 4
1.2 What do we mean by Link-States? 5
1.3 Link-State Algorithm 6
2.0 Shortest Path Algorithm 6
2.1 OSPF cost 6
2.2 Shortest Path Tree 7
3.0 Areas and Border Routers 8
4.0 Link-State Packets 8
5.0 Enabling OSPF on the router 10
6.0 OSPF Authentication 11
6.1 Simple password authentication 11
6.2 Message Digest authentication 11
7.0 The Backbone and area 0 12
8.0 Virtual links 13
8.1 Areas not physically connected to area 0 14
8.2 Partitioning the backbone 15
9.0 Neighbors 15
10.0 Adjacencies 16
10.1 DR Election 17
10.2 Building the Adjacency 17
10.3 Adjacencies on point-to-point interfaces 21
10.4 Adjacencies on Non-Broadcast Multi-Access (NBMA) 21
II.0 Avoiding DRs and "neighbor" command on NBMA 22
11.1 Point-to-point subinterfaces 23
11.2 Selecting Interface Network Types 24
11.2.1 Point-to-multipoint interfaces 24
11.2.2 Broadcast interfaces 27
12.0 OSPF and Route Summarization 28
12.1 Inter-area route summarization 28
12.2 External route summarization 29
13.0 Stub Areas 30
14.0 Redistributing routes into OSPF 34
14.1 E1 vs. E2 external routes 34
15.0 Redistributing OSPF into other protocols 38
15.1 Use of a valid metric 38
15.2 VLSM 38
15.3 Mutual Redistribution 40
16.0 Injecting defaults into OSPF 43
17.0 OSPF Design Tips 45
17.1 Number of routers per area 46
17.2 Number of neighbors 46
17.3 Number of areas per ABR 46
17.4 Full mesh vs. Partial Mesh 47
17.5 Memory issues 47
18.0 Summary 48
19.0 APPENDIX A: Link-State Database Synchronization 49
19.1 Link-State Advertisements 51
19.2 OSPF database example 54
19.2.1 General view of the database 57
19.2.2 Router Links 58
19.2.3 Network Links 59
19.2.4 Summary Links 60
19.2.5 Summary ASBR Links 60
19.2.6 External Links 61
19.2.7 The Full Database 63
20.0 APPENDIX B: OSPF and IP Multicast Addressing 68
21.0 APPENDIX C: Variable Length Subnet Masks (VLSM) 69
Continue reading here: Shortest Path Tree
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