- Open Shortest Path First
- References
    - http://en.wikipedia.org/wiki/OSPF
    - https://en.wikipedia.org/wiki/Link-state_advertisement
    - https://confluence.wiscnet.net/display/KB/OSPF
    - [RFC 1131](https://tools.ietf.org/html/rfc1131)
    - [Dijkstra's algorithm](https://en.wikipedia.org/wiki/Dijkstra%27s_algorithm) (shortest path first)
- [[Routing Protocol]]
    - #[[Link-state [[Routing Protocol]]]]
    - [[IGP]]
- History
    - standardized in 1989
    - Developed largely by DEC
- Terms
    - LSDB (Link State Database): A database that stores LSAs. This is how OSPF builds a picture of the network. A network topology map.
    - LSA (Link State Advertisement): A data structure that describes part of the network.
    - DBD or DDP (Database Description packet): A list of LSAs that a router has. This packet does not contain LSAs, just the headers. This is sent to a neighbor so it can figure out which LSAs to request.
    - LSU (Link State Update): A packet containing LSAs.
    - LSAck (Link State Acknowledgement): Confirms receipt of an LSU.
    - LSR (Link State Request): A packet which requests a list of LSAs that the router has found are missing or out of date based on its neighbor's Dababase Description (DBD).
    - Neighbors: A relationship between routers on the same data link that, once established, allows them to exchange their LSDBs.
- OSPF Areas
    - 32-bit number, can be decimal or dotted-decimal
    - Area 0 (0.0.0.0)
        - Backbone area
        - All areas must connect to Area 0
    - Stub Area
    - Not-so-stubby Area (NSSA)
    - Totally Stubby Area
    - Totally Stubby NSSA
    - Transit Area
- OSPF Router Types
    - IR (Internal Router)
    - BR (Backbone Router)
    - ABR (Area Border Router)
    - ASBR (Autonomous System Border Router)
- OSPF Router Attributes
    - DR (Designated Router)
    - BDR (Backup Designated Router)
- LSA Types
    - Type 1 - Router LSA
        - LSA Originator:: All routers
        - LSA Scope:: Same area
        - Advertises::
            - Itself
            - Links in the same area
    - Type 2 - Network LSA
        - LSA Originator:: DR
        - LSA Scope:: Same Area
        - Advertises::
            - Routers on the same segment
    - Type 3 - Summary LSA
        - LSA Originator:: ABR
        - LSA Scope:: Connected areas
        - Advertises::
            - Prefixes and metrics for connected area
            - Can optionally do route summarization
    - Type 4 - ASBR-Summary LSA
        - LSA Originator:: ABR
        - LSA Scope:: All except stub and NSSA
        - Advertises::
            - Location of ASBRs to other areas. ASBRs flood Type 1 LSA, which are not transmitted to other areas. When an ABR receives a Type 1 LSA form an ASBR, it creates a Type 4 LSA to send to other areas.
            - ![](https://firebasestorage.googleapis.com/v0/b/firescript-577a2.appspot.com/o/imgs%2Fapp%2Fbbchase%2FUoaZ7ipAZx.jpg?alt=media&token=534abe54-41c2-497b-bb0f-988cc9878d3e)
    - Type 5 - External LSA
        - LSA Originator:: ASBR
        - LSA Scope:: All except stub and NSSA
        - Advertises::
            - Information from other routing processes
            - ![](https://firebasestorage.googleapis.com/v0/b/firescript-577a2.appspot.com/o/imgs%2Fapp%2Fbbchase%2FAc9fhpBzT7.jpg?alt=media&token=fad9ca78-ca72-40d2-8179-2d9a8b2bbc0a)
    - Type 6 - Group Membership LSA
        - Meant for Multicast OSPF, Not widely used. 
    - Type 7 -  NSSA External LSA
        - LSA Originator:: ASBR in a NSSA
        - LSA Scope:: Same area
        - Advertises::
            - Mask for a Type 5 LSA in an area that blocks Type 5 LSAs. This is needed when an ASBR is inside a NSSA. When the Type 7 reaches the ABR, it's translated to a Type 5
            - ![](https://firebasestorage.googleapis.com/v0/b/firescript-577a2.appspot.com/o/imgs%2Fapp%2Fbbchase%2FTP4-ZwdBuI.jpg?alt=media&token=6e62dab6-8580-4270-8616-9bfc3b47d1fe)
    - [ ] Pick this up later: http://www.firewall.cx/networking-topics/routing/ospf-routing-protocol/1178-ospf-lsa-types-explained.html
    - Type 8 - External Attributes LSA
        - LSA Originator:: 
        - LSA Scope:: 
        - Advertises::
    - Type 9 - Link Scope Opaque LSA
        - LSA Originator:: 
        - LSA Scope:: 
        - Advertises::
    - Type 10 - Area Scope Opaque LSA
        - LSA Originator:: 
        - LSA Scope:: 
        - Advertises::
    - Type 11 - AS Scope Opaque LSA
        - LSA Originator:: 
        - LSA Scope:: 
        - Advertises::
- Related
    - [[OSPFv3]]
    - [[IS-IS]]
    -