Do OSPF Loopbacks all have to be in the same area?

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Apr 9th, 2014
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I got 7 routers each with a loopback. I also have 3 OSPF Areas...0, 1, and 2

 

Routers 2 and 3 have interfaces in both Area 0 and Area 1

 

Routers 4 and 5 are only in Area 1

 

Routers 6 and 7 are only in Area 2

 

Router1 - 1.1.1.1 /32 loopback 0

Router2 - 2.2.2.2 /32 loopback 0

etc...

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Muhammad Thanveer Wed, 04/09/2014 - 22:44
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No need not be in same area.

Routers in area 1 and 2 can also have a loopback in area 0.

Router in Area 1 can also have a loopback in area 2.

Hope you understand, keep posted.

BR

Thanveer

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Muhammad Thanveer Fri, 04/11/2014 - 23:25
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Hello Joshgormon,

Thank you for the comment and the stars.

By the way I am sorry to say but I haven't received the Stars yet. Better you mark the answer correct.

BR

Thanveer

 

e.ciollaro Wed, 04/09/2014 - 23:57
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Hi,

I agree with  Thanveer Mohd  but be carefull:

  • if you configure a router with interface in Area 0 it's better if it is connect to the whole area 0
  • if you configure a router without interface in Area 0 and loopback it's not on the same area of other interface, loopback will not be  advertised to other routers (you need a virtual link to connect router to area 0)

Finally it's seems you have an area 2 without ABR, is it correct ? And in which area is router 1 ?

 

e.ciollaro

joshgarmon116 Fri, 04/11/2014 - 13:35
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E.ciollaro,

 

Router1 is in Area 0


Router 2 has interfaces in both Area 0 and Area 1

 

Router 3 has interfaces in both Area 0 and Area 2

e.ciollaro Mon, 04/14/2014 - 01:22
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ok, more clear. As I told you can put loopbacks in the same area of other interfaces or not.

On ABR do as you want no problem at all

On other routers (so called internal router), if you configure loopback interf in a different area, the router have to become an ABR so it has to be attached to Area 0, directly or via a virtual link. If not, the loopback will not be advertised.

 

Rate if useful, bye

enrico.

Muhammad Thanveer Mon, 04/14/2014 - 03:20
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Dear friend,

Agree with E.Ciollaro

Please find the scenario as attached, if you put a loopback in area 0  in router 4 you will be able to reach it, if you put loopback in area 5 in R1 you will be able to reach from any where.

While putting the loopback in other areas there will be no issue, but we should put only consider that in the non ABR there should be atleast one loopback in area 0 so that it starts advertising summary routes.

Rating E.Ciollaro 5 stars. 

BR

Thanveer.

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Richard Burts Mon, 04/14/2014 - 06:04
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Thanveer is technically correct that it is possible for a router to have all of its physical interfaces in area 1 and to have its loopback interface in area zero. But remember that because you CAN do something does not necessarily mean that you SHOULD do it. I would suggest that it is quite problematic to have the loopback in area zero but no physical interfaces in area zero.

 

I suggest that the first problem is that if your loopback is in area zero but no physical interfaces are in area zero that you have created a partitioned area zero. One of the principles of OSPF is that backbone area zero routers should be connected to each other. But in this implementation the area 0 of the loopback must go through area 1 to reach other backbone area zero routers.

 

I suggest that the second problem is about summarization. Having the loopback in area zero will allow the router to begin to advertise summary routes. Thanveer seems to view that as a good thing. But I ask the question that if the only routes the router knows about are area 1 routes then why would you want it to send summary advertisements into area 1. Another basic principle of OSPF is that within an area all routers should have the same view of the area. If the router is sending summaries of its subnets then this breaks that principle.

 

To answer the original question - there is no need for all loopback interfaces to be in the same area. And I would go a step further and say that if you have multi area OSPF that it is bad to try to put all loopback interfaces into the same area.

 

HTH

 

Rick

Muhammad Thanveer Mon, 04/14/2014 - 06:36
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Dear Sir,

I completely agree with you Sir, Please forgive me for my bad English, I am explaining this with respect Sir,

In the diagram I attached, R4 is connected to both area1 and area 2, so if I want to reach from area1 router R5 to area 2 router 6 via R4 then I can make R4 as ABR by keeping the loopback in area 0 since loopback can never go down,

Point 1, of mine is to make Questioner understand that ABR is the Router in which atleast one interface is in area 0 and it is not down and, point 2 as you said 

"there is no need for all loopback interfaces to be in the same area. And I would go a step further and say that if you have multi area OSPF that it is bad to try to put all loopback interfaces into the same area"

Point No 3 even though R4 is an ABR, R2 and R3 will never accept the summary LSA from R4 since they have connected interface in Area 0 and the Area 0 is active and they prefer Summary routes from Area 0 interface and moreover Area 0 is inactive in R4 as it has no connected interface in area 0.

Let us consider one scenario thinking that there are no redundant links anywhere in the diagram attached and if the link between R1 and R2 or R1 and R3 goes down then R5 cannot reach to R6. So keeping the Loopback  of R4 in Area 0 makes possible that R5 can Reach R6. 

BR

Thanveer

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