Monday, 1 May 2023

OSPF area types and operations (JUNIPER - REVIEW)

OSPF area types allow networks to be segmented into smaller areas to reduce the amount of routing information that needs to be exchangedThe backbone area is the central area of the OSPF network and must exist in every OSPF network. Standard areas are connected to the backbone area and have their own LSDBs, while stub areas and NSSAs block external routing information to reduce the amount of routing information exchanged between areas. Totally stubby areas block both external and inter-area routes. Routers within the same area share the same LSDB and use it to calculate the shortest path to a destination network.


OSPF defines several types of areas, each with different functions and characteristics below are the OSPF area types and their operations:


Backbone Area (Area 0) - The backbone area (also called Area 0 - area id must be 0 / 0.0.0.0 in 32bit notation) is the central area of an OSPF network and it must exist in every OSPF multiarea network. All other areas must be connected to the backbone area directly, exeption her ei sthe use of Virtual-Link/tunnel. The backbone area is responsible for distributing routing information to other areas, and it forms the foundation of the entire OSPF network.


Standard Areas (Non-Backbone Areas) - Standard areas (also called non-backbone areas) are areas that are connected to the backbone area. Same as Backbone area Standard areas are identified by Area ID which must be different than 0 (or 0.0.0.0). Each standard area has its own link-state database (LSDB) that contains information about the network topology within that area. 


Stub Areas - Stub areas are standard areas that do not receive external routing information. External routing information is information about networks outside the OSPF domain. Stub areas are used to reduce the amount of routing information that needs to be exchanged between areas. Instead of receiving external routing information, stub areas use a default route to forward traffic to networks outside the OSPF domain. Stub areas can be configured as totally stubby areas, where no inter-area or external routes are allowed, or as not-so-stubby areas (NSSAs), which allow the injection of external routes but with some limitations.


Totally Stubby Areas - Totally stubby areas are a type of stub area that not only blocks external routes, but also inter-area routes. This means that routers within the totally stubby area only know about routes within their own area and the default route to reach networks outside the OSPF domain.


Not-So-Stubby Areas (NSSAs) - NSSAs are a type of stub area that allow the injection of external routes, but with some limitations. Unlike standard areas, NSSAs do not receive external routing information directly. Instead, an NSSA border router (ASBR) injects external routes into the NSSA as Type 7 LSAs, which are then translated into Type 5 LSAs by an NSSA Area Border Router (ABR) before being propagated into other areas.

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