cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
3707
Views
13
Helpful
32
Replies

Multiple ABRs in stub area

GillieLucent
Level 1
Level 1

Hi all,

Attached is my topology.

I am injecting default route into area 1 from R1 with cost 10 and from R2 with cost 40.

In that case, will both default route be in area1's database ?

OR let me know, which default route does R5 takes and which one does R4 takes to reach external destination .

Thanks,

Vijaybabu

32 Replies 32

cjnwodo
Level 1
Level 1

Hi,

Both default routes should be in the OSPF database, however only the one from R1 [with the lower cost] should make it into the routing table. So both R4 and R5 should use R1.

Hi,

Can someone explain bit more ?

It would be better if you explain how R2's default route reaches R5 and vice versa.

Thank,

Vijaybabu

Hi,

Before I do so, can I ask a question? Are the physical links in your diagram the only ones, in the network? or do R4 and R5 have other connectivity to each other or do they [R4 & R5] both connect to R1 and R2?

Hi,

Also what is the physical link between R1 and R2 and what area is that link in?

Hi,

Thanks for your prompt response.

There is no connectivity between R4 & R5. R4 is connected only to R2 and R5 is connected only to R1. The connection between R1 and R2 is broadcast network and its in Area 0.

Is R3 connected to only R2?

Narayan

R5 would use R1. R4 would use R2.

Both R5 and R4 would see only one ABR each. You can verify that by issuing the command 'show ip ospf border-routers'. There will be only one option in your setup for R5 and R4 and thus choosing the route based on OSPF metric doesn't arise.

HTH

Sundar

Hi Sundar,

But both R5,R5 will receive Router LSAS from both R1 and R2, so both R4 and R5 will come to know there are two ABRs in the area. In that case, it should choose ABR which advertises least default cost, isn't ?

Please explain elaborately.

Thanks,

Vijaybabu

Vijaybabu,

R5 will not receive LSAs from R2. The reason for that is you have a partitioned area1. What you are saying would be true if the link between R1 and R2 is in area 1. As far as R1 and R2 is concerned they consider area1 as two separate areas.

HTH

Sundar

Hi Sundar,

Thanks for your clearing my doubt.

I really didn't think about that, I just enabled neighbourship without thinking in which area its needed.

So, there is no use of the link between R1 and R2 in area 0.

Can you explain me if area 1 is not a stub area ?

Thanks,

Vijaybabu

Area 1 is a normal area and not OSPF stub area. If you want to make area1 as stub you would have to configure all the routers in area 1 as stub area. As I said before if you configure the link between R1 and R2 to be area 1 then both R4 and R5 would receive LSAs from both ABRs.

HTH

Sundar

Hi Sundar,

Actually, in my topology area 1 is a stub area.

Now, I want you to explain the behavior if area 1 is not a stub area .

Thanks,

Vijaybabu

It shouldn't make any difference in the current behavior whether it's normal or stub area. If you desire you can make R5 and R4 a stub , a totally stub would even be a better option, in their own area.

HTH

Sundar

Hi Sundar,

You are answering well, but not trying to explain.

We are not experts, we are learners.

Explain me in the below scenario :

In the topology diag, I have made a connection between R4 and R5, then what could happen ?

Thanks,

Vijaybabu

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community:

Innovations in Cisco Full Stack Observability - A new webinar from Cisco