OSPF Area Addition - Design Question

[lxcollin1]

Hello,

I have a design question regarding OSPF. I am looking to add a new ospf area (1). The area will live on two Core routers and two Distribution routers. Can you please look at the attached Pics and tell me which design is better.

I would like to be able to connect Core-01 to Dist-01 and Core-02 to Dist-02 with a connection between Dist-01 and Dist-02, but this will result in a discontiguous area, correct?

Thanks,

Lee

[lxcollin1]

I forgot to add the Pics. I have attached them now.

[pkhatri]

Hi,

Looking at your two options, both are perfectly valid and neither results in a discontiguous area.

I would tend to prefer the second option (pic2) as it is the more common design where distribution routers are dual-homed to the core routers. In addition, with this option, the loss of the direct link between the two distribution routers would result in fewer hops through the core routers in order to reach the partitioned parts of that area from the other partition.

Also, with this second option, traffic the distribution routers can travel to the core router where the destination is directly, without having to hop through the other core router...

Pls do remember to rate posts.

Paresh

[lxcollin1]

Thanks for your reply Paresh...

Question: is it common to have two parallel areas between core routers as depicted in Pic 1? Also, is there any drawbacks to running this area as a Totally NSSA?

Thanks!!

[pkhatri]

I would say that the more common design is to have just backbone area links between the core routers. But there is no real issue with having an area 1 link between them...

If I were you, I would not make the area a totally NSSA. Here are my reasons for that:

- you will get sub-optimal routing out of the area since you have two ABRs and each distribution router will pick the closest one of them to get out to the backbone even though it may be more optimal to use the other one

- in an NSSA case, one of the two ABRs will be designated as the NSSA translator, which means that if you are doing summarisation on the ABRs, all traffic destined for these summarised routes will be drawn to the area through that one ABR.

Paresh

[lxcollin1]

So are you saying that all traffic destined to area 1 will always use the same ABR? Does this apply only if summarization is configured?

Also, if I have the ABRs generating the default route, will the two Dist routers be able to use the default route with the least cost, or does all traffic need to flow to the NSSA translator? In other words, can I have both ABRs active for the NSSA?

FYI... the Dist routers do not need to know routes outside of the NSSA area so because all outside routes are learned via BGP. The Dist routers will not participate in BGP.

thanks for your help!!

[pkhatri · Accepted solution]

It's only summarised external routes that will exhibit this behaviour - if that is not something you do, then this is not an issue for you.

The NSSA translator role will NOT influence which ABR is used to exit the area - the closest one will always be used. So yeah, both ABRs will be active.

Looking at your requirements, I don't believe there will be any issues if configure the area as a total NSSA.

Paresh

[lxcollin1]

Sounds good. I appreciate your help!!!

[lxcollin1]

Sounds good. I appreciate your help!!!

[pkhatri]

Hi Mate,

I just had another thought about this design (which you have proabably already considered).

If you are going to be injecting a default route into the totally NSSA area, intending that the rest of the OSPF domain uses it, beware that the type-3 default route generated by the NSSA ABRs will override this external default, rendering it unusable.

Again, this may not impact your requirements at all...

Paresh