OSPF Areas (Part 2) - Backbone and Normal Areas

Links:

RFC 2328 – OSPF Version 2
RFC 2740 – OSPF for IPv6

Backbone area (Area 0)

Let’s start with the Backbone area. The backbone area is just as the name implies the backbone of the OSPF domain. Think of the backbone area as the hub for OSPF. The backbones main responsibility is to distribute routing information (LSAs) between non-backbone areas.


Breakdown of Backbone Area:

• Backbone = area  0 (or area 0.0.0.0)
• Must be contiguous and not split up
• All area ABRs must connect  to the backbone 
• Virtual links are used when areas are not physically connected
• Cannot be configured as stub or NSSA area type
• Redistribution allowed into the backbone area
• All LSA types are allowed and propagated

OSPF Areas - Index

OSPF Areas (Part 1)
OSPF Areas (Part 2) - Backbone and Normal Areas
OSPF Areas (Part 3) - Stub
OSPF Areas (Part 4) - Totally Stubby
OSPF Areas (Part 5 ) - NSSA
OSPF Areas (Part 6 ) - Totally NSSA

OSPF Areas (Part 6) - Totally NSSA

Links:
RFC 2328 – OSPF Version 2
RFC 3101 - The OSPF Not-So-Stubby Area (NSSA) Option
RFC 2740 – OSPF for IPv6
Cisco - OSPF Not-So-Stubby Area (NSSA) 

Breakdown of the Totally NSSA Area:

• Redistribution allowed by ASBRs
• NSSA ASBRs generate Type-7 LSAs for external routes
• Appear as N1 or N2 external
• ABRs convert Type-7 LSAs to Type-5
• Blocks Type-5 External, Type-4 ASBR and Type-3 Summary LSAs
• Only ABRs are configured for totally NSSA 
• Internal routers configured as NSSA to form an adjacency
• N-Bit is set to 0 in Hello packet
• Cannot be backbone area (area 0)
• Virtual links are not allowed to transit

At this point you should already have a good idea of what a Totally NSSA area is. Basically a totally NSSA area is the combination of, wait for it... wait...