📘 PCNSE OSPF LSA Types, Area Types, and Redistribution

1. OSPF LSA Types

Type 1 – Router LSA: Generated by all routers to describe their directly connected links within an area.
Type 2 – Network LSA: Generated by the Designated Router (DR) to describe the routers attached to a broadcast or NBMA network.
Type 3 – Summary LSA: Generated by Area Border Routers (ABRs) to advertise networks from one area to another.
Type 4 – ASBR Summary LSA: Generated by ABRs to advertise the location of Autonomous System Boundary Routers (ASBRs).
Type 5 – AS External LSA: Generated by ASBRs to advertise external routes into the OSPF domain.
Type 7 – NSSA External LSA: Generated by ASBRs within Not-So-Stubby Areas (NSSAs) to advertise external routes; converted to Type 5 LSAs by ABRs when exiting the NSSA.

Backbone Area (Area 0): The core of an OSPF network; all other areas must connect to it.
Stub Area: Blocks Type 5 LSAs; ABR injects a default route (Type 3 LSA) into the area.
Totally Stubby Area: Blocks Type 3 and Type 5 LSAs; ABR injects only a default route. This is achieved by disabling the "Accept Summary" option on the ABR interface in Palo Alto Networks firewalls [Source] .
Not-So-Stubby Area (NSSA): Allows an ASBR within the area to inject external routes as Type 7 LSAs; ABR translates these to Type 5 LSAs when advertising to other areas.
Totally NSSA: Similar to NSSA but also blocks Type 3 LSAs; ABR injects only a default route and translates Type 7 LSAs to Type 5 LSAs for external areas.

Redistribution allows routes from other protocols (e.g., BGP, RIP, static) to be advertised into OSPF. Key considerations:

ASBR Role: The router performing redistribution becomes an ASBR.
LSA Types: External routes are advertised as Type 5 LSAs in standard areas or as Type 7 LSAs in NSSAs.
Route Maps: Use route maps to control which routes are redistributed and to set attributes like metrics and tags.
Default Routes: When redistributing, ensure that default routes are appropriately injected if required, especially in stub or NSSA areas.

Understand the differences between OSPF area types and their impact on LSA propagation.
Be familiar with how Palo Alto Networks firewalls implement OSPF, including the configuration of stub, totally stubby, and NSSA areas.
Know how redistribution works in OSPF and the role of ASBRs.
Be aware of how default routes are injected into different area types.
Understand the significance of LSA types and their scope within the OSPF domain.