CCNA Basics: Quick reference on OSPF for Network Engineers

Today I am going to discuss about the basic and the quick reference on OSPF protocol. It is very useful for the Network engineers to understand the basics of OSPF with the below information for the interview purposes.

All OSPF Routers Multicast Address – 224.0.0.5
All OSPF DRs Multicast Address – 224.0.0.6

OSPF Packet Types

• Type 1 – Hello – Used to discover & maintain neighbors
• Type 2 – DBD – (Data Base Descriptor) Used in adjacency formation process to exchange LSAs
• Type 3 – LSR – (Link State Request) Used when LSDB becomes stale or is missing an LSA
• Type 4 – LSU – (Link State Update) Contains LSAs
• Type 5 – LSAck – (Link State Acknowledgement) Acknowledges LSAs
  An individual link-state acknowledgment packet can contain an acknowledgment for a single link-state update packet or for multiple link-state update packets. 

Fig 1.1-OSPF Sample Topology

OSPF Adjacency States

• Down – Initial state, waiting for start event
• Init – Hello sent but bidirectional communication not established
• 2Way – Hello received, bidirectional communication established
• ExStart – Routers negotiate who in in charge of LSDB sync
• Exchange – Exchange LSA headers, if missing data send LSR
• Full – LSDBs fully sync’d

OSPF Router Types

• DR – Designated Router – Represent multiple routers on a broadcast link
• DR forms Full adjancies with all DROthers and the BDR
• BDR – Backup Designated Router
• BDR forms Full adjancies with the DR and all DROthers, does not advertise learned link state info
• DROther – Other routers on a broadcast link
• DROthers form 2Way adjacencies with each other
• ABR – Area Border Router – Router with links to at least 2 different areas
• ASBR – Autonomous System Boundary Router – Injects information from outside the OSPF AS
• Backbone Router – Any router with a link to the Backbone Area
• Internal Router – Any router with all links in the same area

OSPF Area Types
*Multiple areas are used to shrink LSDB size and isolate troubles

• Backbone Area (Area 0) – Distributes routes between different areas
• Stub Area – No AS External LSAs (Type 4 & 5) are flooded, ABR floods default route
• Totally Stubby Area – Receives only default route (No Type 3, 4, or 5)
• Not-So-Stubby-Area – Can receive and leak external routes in area, but external routes from other areas are not flooded (No Type 4 & 5)

OSPF Route Types

• External – Redistributed from other protocols (LSA Type 5 & 7)
• Inter-Area – (Summary Routes) Originate from other areas (LSA Type 3 & 4)
• Intra-Area – (Internal Routes) Originate within same area (LSA Type 1 & 2) 

OSPF LSA Types

• Type 1 – Router – Intra-Area Adjacencies (DROther to DR)
• Type 2 – Network – Describe Broadcast Segment (DR to DROthers)
• Type 3 – Summary – Area Summary Sent by ABR to other areas
  *OSPFv3 – Inter-Area-Prefix-LSA
• As re-injected into other areas LSA type doesn’t change
• Type 4 – ASBR Summary – ASBR Description Sent by ABR to other areas
  *OSPFv3 – Inter-Area-Router-LSA
• As re-injected into other areas LSA type doesn’t change
• Type 5 – AS-External – Sent by ASBR describing prefixes from other protocols
  *OSPFv3 – AS-External-LSA
• As re-injected into other areas LSA type doesn’t change
• Type 6 – Used by Multicast OSPF, deprecated in OSPFv3
• Type 7 – NSSA External – Sent by ASBRs in NSSAs
• Translated to Type 5 LSA by NSSA ABR
• Type 8 – External Attributes (Intended to mimic capability of iBGP)
  *OSPFv3 – Link-LSA
• Type 9 – Opaque LSA (Link Scope)
  *OSPFv3 – Intra-Area-Prefix-LSA
• Type 10 – Opaque LSA (Area Scope)
• Type 11 – Opaque LSA (AS Scope)