2

Considering This Topology:

enter image description here

Which is a part of the Practical Networking youtube educational channel, we can see three areas.

The teacher said that all traffic should go through Area 0 to keep a Hub and Spoke design, and to avoid routing loops.

My questions:

1- Isn't OSPF a loop-free protocol? Shouldn't it avoid loops automatically?
2- What happens if we add new router R8 to:

  1. Area 88 and Area 99?
  2. Area 88, Area 99, and Area 0?

I tried to find an answer on the Internet, but it is always around the subject and not to the point answer. I also tried to create a Packet Tracer lab to simulate the topology but I couldn't find a 4-port router to simulate the R03 router.

As I am a novice, I will appreciate simplified answers.

2 Answers 2

6

1- Isn't OSPF a loop-free protocol? Shouldn't it avoid loops automatically?

OSPF is loop-free precisely because all inter-area traffic must go through area 0

2- What happens if we add new router R8 to:

Area 88 and Area 99?

The router will not advertise area 99 routes into 88, and vice versa. Note that any device using this router as a gateway will be able to reach the other area, as the router has routes for both.

Area 88, Area 99, and Area 0?

The router becomes another ABR for 88 and 99, but again does not advertise 88 into 99 (and vice versa).

3

1- Isn't OSPF a loop-free protocol? Shouldn't it avoid loops automatically?

Yes, and it will, if you follow it's design rules. Violating the rules might make it hard/impossible to detect a loop. At the Area Border between 0 and non-0 areas, summarization can occur, and summarization must be done with care to prevent loops.

  1. What happens if we add new router R8 to:

    1. Area 88 and Area 99?

That would be a wrong setup. Any Area Border Router (ABR) must be connected to Area 0 with at least one interface. I might participate in routing in either/both areas, but it will not announce prefixes from one area to the other. Therefore, other intra -area routers in 88 (resp 99) will not forward packets for area 99 (resp. 88) toward R8.

  1. What happens if we add new router R8 to:

  2. Area 88, Area 99, and Area 0?

That would be valid, and R8 would be just another regular ABR.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.