Date of Issue: 08-01-2023 Rate this Study Guide

OSPF Part 2: Using OSPF in Hierarchical Systems

by Howard Berkowitz

Hierarchy in OSPF
    Selecting Area Identifiers
    There is enough ugliness in the world
Basic OSPF Components
  Special Considerations for Area
    Don't Be Seduced by a Single Area
  Area Sizing
  Introducing the ABR
    A Key Difference between OSPF and ISIS
Basic Multiple Area Configuration
  Interior Router Configuration
  Simple ABR Configuration
  More Complex ABR Configuration
  Backbone Router Configuration
  Introducing the ASBR
  Types of Externals
  OSPF and Defaults
    Redistributing Static Default into OSPF
    Redistributing Default into OSPF from Another Dynamic Routing Protocol
    Externals from the Internet
  Applications of Type 1 and Type 2 Externals
    Load-Sharing ISP Connection
    Primary-Backup ISP Connectivity
  Hierarchical versus Mutual Importing and Exporting
  LSAs and LSA Propagation
    Wisdom from fellow ZoneMaster Peter van Oene
Summarization and Aggregation
  How to Summarize
    Another Reminder of Different Kinds of Masks
  Aggregating Externals
  Advertising Holes
Area Types: Limiting LSA Propagation
  Basic Stub Areas
    Restrictions of Basic Stub Areas
  Totally Stubby Areas and Closest Exit Routing
    Restrictions of Totally Stubby Areas
  Multiple ABR Issues and Asymmetrical Routing
  Not-so-stubby Areas
    Restrictions in NSSAs
  The Totally Stubby and Also Not-so-stubby Area
Breaking Hierarchy
  Manual Traffic Engineering
  Tunnels and OSPF
    Virtual Links
    Healing Backbone Partitions across a Non-OSPF Domain
    Linking Disconnected Areas
Learning More

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