I have a question regarding a hierarchical design of a campus network.

Take a look at this image: OSPF Area diagram

If I understand correctly the links between each Area Border Router and BBone router are point-to-point Layer 3 connections. With every point-to-point link in it's own subnet (probably a /30).

But I was wondering if, for example, links going from ABR-1 & ABR-2 to BB-R1 go to a switch, would this break the "L3 down to the access level" design idea? If so, why's that?

It would look like: enter image description here Switch now drawn- The green lines represent one subnet, and the red one a different subnet.

Thanks for your help!

1 Answer 1


The idea behind "L3 to the access layer" is to avoid blocked links that would occur when STP blocks L2 loops, and allow multiple paths for increased bandwidth.

In your second drawing (assuming the link between the two ABRs in area 120 is not in the same VLAN), you don't have any L2 loops, so you don't have that problem.

So there's no significant disadvantage in having the links in the same VLAN. Technically you do have a designated router to deal with, but that shouldn't cause you to change your design.

However, if you're using an external switch (your backbone and area 120 ABRs connect to a common switch, each with a single connection), then you lose the bandwidth (and redundancy) that you would have with a separate physical link to each router.

(Edited to add designated router info)

  • Perect makes sense. I just not quite get the idea of what's the device pictured in the first image? Is that a Layer 3 Switch? And, if that's the case, are the connections between the ABRs in are 120 with the BB routers all VLANs? Just like this vlan 10 ABR-1 <-> BB-R1 and vlan 20 ABR-1 <-> BB-R2 and vlan 30 ABR-2 <-> BB-R1 and vlan 40 ABR-2 <-> BB-R2 ?
    – Gero
    Commented Jun 24, 2015 at 20:09
  • 1
    There is a disadvantage to having the links in the same VLAN. Doing so would then require the DR/BDR election process to be in effect (not necessary on properly configured point-to-point L3 links) and may involve that process in any troubleshooting when you experience problems. Not saying there aren't any advantages, but did want to point out there are disadvantages since you said there weren't any.
    – YLearn
    Commented Jun 24, 2015 at 21:37
  • Ok. You are right of course but I would consider that a minor point.
    – Ron Trunk
    Commented Jun 24, 2015 at 21:40
  • How would the L3 Switch at the core (BB-R1) needs to be configured to handle that amount of interfaces? Does high end routing devices have that amount of ports to plug in point-to-point links?
    – Gero
    Commented Jun 24, 2015 at 23:33
  • @Gero There's no special configuration for the core switch. You probably want to ensure that it is the designated router (ip ospf priority x), but other than that, it's the same regardless of the number of ports. Yes, large routers (and switches) have lots of ports.
    – Ron Trunk
    Commented Jun 25, 2015 at 11:40

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