Brief answers for a brief questions about OSPF

1. Why area 0 is required? Per RFC 3509 two non-backbones area cannot communicate and to ensure a loop free topology in a multi area design, especially with redundant paths , we need a central point and a reference to calculate the best path without a risk of routing loop, here comes the ABR which a special router that interconnects a non-backbone area with a backbone area, all inter-area traffic must go through the backbone area.

In reality, OSPF is a link-state routing protocol only within an area (intra-area); but almost a distance-vector routing protocol between areas (inter-area).

One of the advantages of link state protocols is that the link state database provides a “view” of the entire network but only within the area. Within the same area every OSPF router floods information about itself, its links, and its neighbors to every other router. From this flooded information each router builds an identical link state database. Each router then independently runs a shortest-path-first calculation on its database and calculates the best path to each destination.

When an OSPF domain grows large, the flooding and the resulting size of the link state database becomes a scaling problem. The problem is remedied by breaking the routing domain into areas.

When an ABR receive the LSA Type 1 and LSA Type 2 within the area, it will only send the reachability information through the LSA Type 3to another area. ABR hides the topology information and only reachability information sends between the areas.

To prevent routing loops, areas must be connected to the backbone area 0. All LSAs Type 3 must therefore pass into or out of area 0 when multiple areas are in use, whereas type 1 and 2 LSAs are confined to the local area. In other when we have multiple ABRs, an ABR ignores an LSA Type 3 learned through a non-backbone, this called as split-horizon inter-area loop prevention.

2. Can we do summarization of LSA-1 ? you can summarize the subnets carried in the LSA Type 1 to a single LSA Type 3 only. summarization is only possible in a multi area design.

3. In NSSA, external routes enters as LSA-7. But, if LSA-7 is not required, what is the command to stop LSA-7 ? if you need an NSSA area but you do not want to advertise a Type-7 LSA, to stop a Type-7 LSA to traverse the NSSA area, use the “nssa-only” keyword in the redistribute command. The “nssa-only” instructs the ASBR to clear the P-bit in its Type7-LSA. The P-bit = (P – propagate) is only used in type-7 LSAs to tell the ABRs to translate that LSA Type 7 into an LSA Type 5.

4. In NSSA, we have one ASBR & two ABR’s. which ABR will do LSA-7 to LSA-5 conversion ? the highest ABR router ID will be the translator per RFC 1583 and 3101, but if you want to force an ABR to be a translator regardless the router ID selection, make sure the RFC is 3101 enabled (this is the default) and execute the area X nssa translate type7 always command.

5. In NSSA, while receiving routes through LSA-7, can we receive selected subnets or stop few subnets from receiving ? you can control the advertised routes through Type-7 using the summary-address command with the “not-advertise” keyword, the component networks remain hidden from other networks. or using the route-map associated with the redistribute command.

6. In OSPF, in Broadcast network type, if all the routers are added with priority=0, then what will happen, will the routers make adjacency or not ? a priority 0 means the router is not eligible to participate in the DR or BDR election , this means, two neighbors routers with a priority 0, the adjacency will stop at the 2 WAY state and no routes are exchanged, no LSAs exchanged.

7. In OSPF, why E1 routes are preferred over E2 routes ? What is the logic ? why E1 route is preferred over E2 route, E1 route uses lowest redistributed cost + lowest cost to reach ASBR we have hot potato + cold potato routing so packet will reach destination as quickly as possible. E1 considers only the redistributed metric and ignores the internal the metric to reach ASBR, this behaviour is called cold potato routing.

8. In OSPF, what is forwarding address ? is it present in LSA-1 packet in communication ? The Forward Address is a concept related to external route through a Type-5 under certain conditions and through a Type-7 ( mandatory). The idea behind this FA is to avoid a suboptimal routing in some design to reach the external routes, in other words it helps a router in an area to have a view of a hidden topology in other area, for example in NSSA, the ASBR is no longer visible from other areas, this can cause a suboptimal routing in the OSPF domain to reach the external subnets from other areas, and to make the ASBR visible, the Forward Address which is an IP address that belongs to the ASBR will help other routers in other areas to locate and calculate the shortest path to the ASBR through an inter-area route toward this Forward Address, thus the best path to the external subnets.

Published by:

Redouane MEDDANE

Redouane MEDDANE is Cisco Instructor CCSI #35458, 3xCCNP Collaboration, Security and Enterprise and he a published author of some of the most important OSPF Protocol, Security and Collaboration books in the world titled OSPF Demystified With RFC, Network Security All-in-one, and Dial Plan and Call Routing Demystified on CUCM. He is also a blogger at and writes articles about collaboration and security to demystify the most complex topics. His books are known for their technical depth and accuracy especially the OSPF Demystified With RFC book, which is considered as the best OSPF book in the world and named "One of the best OSPF ebooks of all time" by BookAuthority It gives you a hint at the ability to explain complex topics with remarkable ease. He worked as a Cisco Instructor and consultant indifferent Cisco Learning Partner and awarded twice as Cisco Distinguished Instructor Award and Cisco Security Instructor Excellence Award on 2018 and 2019, and Cisco Collaboration Instructor Excellence Award on 2020. The Distinguished Instructor Award recognizes the top 5% of Cisco's most influential CCSI's who provide the highest quality training experience and demonstrate the best overall instructor performance across multiple Cisco technologie and Instructor Excellence Award recognizes the top 25% of elite CCSIs being recognized for delivering top quality training and maintaining high customer satisfaction in their field of expertise.

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