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OSPF - four routers topology

Unanswered Question
Feb 26th, 2012
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Hi,

There are four locations and four routers, two are head office (cat6k) and two are branch locations (cat3750), connected in a ring topology

HO1 === HO2

I. I

BR1 --- BR2

What is the best approach to the ospf configuration? Should all routers be in area 0 ? Or should the branch routers be in a stub/totally stub area?

Thanks

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Reza Sharifi Sun, 02/26/2012 - 07:35
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Hi,


If your are not carrying a lots of routes, for just 4 routers, I would put them all in one area (area 0).


HTH

swordcrowned Sun, 02/26/2012 - 08:10
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Hello,


I agree with Reza.  With only four routers, putting them into one area would be just fine.  However, if you plan on growing the network with many more branch offices, you may want to start creating stubby networks in different areas.


Bryan Hefner

rjuchta Sun, 02/26/2012 - 10:33
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Ok, I should add that this is only a part of bigger network and on the branch routers there are about a few hundreds of routes. So the stub option seems to be more recommended .?. In such case should there be two different stub areas or one common for both branch routers?

Reza Sharifi Sun, 02/26/2012 - 11:39
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If you only have a few hundred routes, one Stub area and an area 0 should be enough.

Richard Burts Sun, 02/26/2012 - 16:34
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This question is getting more difficult to answer well.  


The original post asks what seems to be a fairly simple question - with 4 routers in a ring topology what is the optimum architecture (one area or multiple areas).  And he got good answers from Reza and Bryan. But then he says that this is only part of a bigger network - but he does not tell us anything about that other network. He only mentions the number of routes on the branch routers.


In my experience with OSPF it matters much less how many routes are in the routing table and matters much more how many peers, and how stable the peers are. Since we do not know these dimensions, it is difficult to give good answers about the big picture. But in terms of the part that has been described to us, I agree with Reza and Bryan. I believe that having these 4 routers in a single area is the optimum solution. I would not necessarily say it needs to be area 0 and it might be some other area - depending on the big picture, which we do not have.


In addition to the reasons given by Reza and Bryan I would suggest that the 4 routers in a ring was probably done that way to provide redundancy, so that wither branch router could access either head office router. Achieving this is easy when they are all in the same area and is pretty impossible if there are multiple areas for the 4 routers.


HTH


Rick

rjuchta Mon, 02/27/2012 - 00:40
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Sorry for not giving the full picture at the beginning. The branch routers have only 2 ospf neighbours, head office routers - 10-13 neighbours. And they are pretty stable.

Still I'm just wandering what are the best practices in such topologies, furthermore if Cat.3750 switches are reliable enough to deal with hundreds or more of routes.

Thanks

Richard Burts Mon, 02/27/2012 - 06:26
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I would think that a 3750 would be reliable enough with several hundred routes.


In terms of best practices I would offer these principles as guidance:

- as the number of peers gets larger it may become more helpful to implement multiple OSPF areas.

- if some links consistently have stability issues it may be helpful to put them into a separate OSPF area.

- if the network has a cluster of layer 3 devices and that cluster has a small number of links to the rest of the network it may be helpful to put that cluster into a separate area. (it is especially helpful if the IP addresses used in that cluster form a summarizable block of IP addresses).

- sometimes topology and function help to indicate when multiple OSPF areas are helpful. For example my suggestion that the ring topology and the desire for redundancy would indicate that they should be in a common area.


HTH


Rick

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