OSPF LSA

From IPFlow Netflow Collector

• LSA #1 (router) contain router ID and the list of prefixes which are not on a shared media (ie, no DR election)
• On a shared segment, non-DR routers form an adjacency with DR and BDR routers. The DR sends LSA #2 (network) to list which routers are members of the segment.
• External routes (LSA #5) are not present/allowed in stub areas.
• When an external route (LSA #5) is sent by an ABR router to another area, it sends the router ID of the originating router as LSA #4.
• In Stub areas, inter-area routes and a default route are emitted as LSA #3 by the ABR.
• In Totally Stub areas, only a default route is emitted as LSA #3 by the ABR (this is the only LSA #3 allowed).
• In NSSA, External routes are present as LSA #7 and are translated to LSA #5 by the ABR routers.

Table of contents

1 Source: Normal area -> Destination: normal area 2 Source: Backbone area -> Destination: classic stub area 3 Source: Backbone area -> Destination: totally stub area 4 Source: Backbone area -> Destination: NSSA 5 Source: Stub area (normal stub, totally stub, or NSSA) -> Destination: Backbone area 6 Source: NSSA -> Destination: Backbone area

Source: Normal area -> Destination: normal area

• LSA #1 (router ID) and LSA #2 (network) are converted by ABR to LSA #3 (inter-area)
• LSA #5 (external routes) are transferred, and LSA #4 (Summary ASBR) are generated by ABR (router ID of the originating router)
• Source or destination area must be the backbone area.

Source: Backbone area -> Destination: classic stub area

• LSA #1 (router ID) and LSA #2 (network) are converted by ABR to LSA #3 (inter-area).
• A default route is generated by the ABR as LSA #3.
• LSA #4 and LSA #5 are not sent since not allowed in stub areas.

Source: Backbone area -> Destination: totally stub area

• A default route is generated by the ABR as LSA #3. This is the only LSA #3 allowed in the totally stubby area.
• LSA #4 and LSA #5 are not sent and not allowed.

Source: Backbone area -> Destination: NSSA

• LSA #1 (router ID) and LSA #2 (network) are converted by ABR to LSA #3 (inter-area).
• A default route can be announced as LSA #7

Source: Stub area (normal stub, totally stub, or NSSA) -> Destination: Backbone area

• LSA #1 (router ID) and LSA #2 (network) are converted by ABR to LSA #3 (inter-area).
• LSA #4 and LSA #5 cannot exist in stub areas.

Source: NSSA -> Destination: Backbone area

• LSA #1 (router ID) and LSA #2 (network) are converted by ABR to LSA #3 (inter-area).
• LSA #7 are converted to LSA #5 by the ABR.