Network design - Layer 3 boundary (core/collapsed core vs. edge/access)

Posted on 2012-09-10
Last Modified: 2012-09-12
I'd like to get some input regarding network infrastructure design, and L2/L3 boundary recommendations.

Let's assume the customer has a collapsed core w/ X IDFs. The current deployment is Layer 2 everywhere, w/ all routing coming back to the core stack/chassis. (They're not currently IP voice/video, but we'd like to design a solution that will support this, and efficient inter-data/v/v routing for collaboration purposes.)  Each IDF/building will have Y endpoints.

At which point do you recommend a Layer 3 (L3) boundary at each IDF vs. an all L2 solution?  Specifically, we could position a single L3 switch at each IDF (like a Cisco 3560), with 2960 L2 switches within the building, to 1) control b/m-cast traffic, and 2) to facilitate inter-VLAN routing (between data/voice/vid VLANs) for more efficient collaboration traffic. Without a L3 switch at each IDF, all this b/m-cast and routing traffic would go back to the core.

I'm hoping/looking for design recommendations - case studies/best practices, whatever. Customer-facing, I need to present real numbers for a L3 IDF switch vs. pure L2 for whatever number of ports. (Again, keep in mind that we need to design the solution to comfortable support IP voice/video in the future.)

Thanks, and reference docs/links are always appreciated.
Question by:cfan73
    LVL 6

    Expert Comment

    For me it comes down to how much intervlan routing is necessary. For example, how much traffic from a VLAN terminating at a particular idf would be rerouted to another host on another VLAN serviced by the same idf. In this example, it would make sense to route at the IDF.

    However if the majority of the traffic is going to the datacenter IDF or the Internet, the routing at the IDF is only going to slow things down as they will have to be routed again.


    Author Comment

    In this particular case, the majority of the traffic will be traversing the core. Once they roll out IP telephony and collaboration integrations (such as desk phone control from their PC, etc.), then there will be routing required between the data, voice and video VLANs, but that would be fairly light traffic.

    So, if routing isn't really the concern in this case, I'm wondering if there's a guideline for how many end stations would warrant having L3 at the edge simply to localize broadcast/multicast traffic.

    Thanks again
    LVL 6

    Accepted Solution

    With voice and video, your core is still going to house the servers. The only exception would be internal voice communications, but depending on your deployment, they may still be proxyed through a gateway.

    The number of devices does not determine the need for layer three switches in the IDF. They way they communicate is the determining factor.

    Remember that every time a routing decision must be made, traffic slows. I know layer three switches can route at wire speed, but I'm guessing you'll want some kind of dynamic routing protocol and security which slows things down. The more device that route, hops, the slower the transmission.

    Ask yourself this question, how many VLANs are represented on switches in the average network closet. Then ask yourself, how often does traffic go directly from one of those VLAN to another VLAN represented in the same closet.

    If the second answer is less than 10%, and it is indicative of 50% or more of your network closets, then you should consider routing in the closet. Otherwise, keep it in the core. You'll enjoy much better performance.


    Author Closing Comment

    Thank you - that's a good reference point.

    Write Comment

    Please enter a first name

    Please enter a last name

    We will never share this with anyone.

    Featured Post

    How to improve team productivity

    Quip adds documents, spreadsheets, and tasklists to your Slack experience
    - Elevate ideas to Quip docs
    - Share Quip docs in Slack
    - Get notified of changes to your docs
    - Available on iOS/Android/Desktop/Web
    - Online/Offline

    I wrote this article to help simplify the process of combining multiple subnets. This can be used for route summarization also but there are other better ways to summarize routes, This article is a result of questions I participate in here at Ex…
    Implementing Avaya's One-X portal is pretty painless, until you want to deploy this to the Android and iPhone clients when these clients are outside of your network. The clients will also work within your local network. Here is our experience and so…
    After creating this article (, I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
    After creating this article (, I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…

    737 members asked questions and received personalized solutions in the past 7 days.

    Join the community of 500,000 technology professionals and ask your questions.

    Join & Ask a Question

    Need Help in Real-Time?

    Connect with top rated Experts

    19 Experts available now in Live!

    Get 1:1 Help Now