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
- Dijkstra’s 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- LSR (Link State Request): A packet which requests a list of LSAs that the router has found are missing or out of date based on its neighbor’s Dababase Description (DBD).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]]