Blessed by Gartner: Stretched VLANs Make Little Sense

One of my readers recently pointed me to a blog post written by Andrew Lerner from Gartner describing the drawbacks of stretched VLANs.

TL&DR: He’s saying more-or-less the same things I’ve been preaching for years. Now I can put Blessed by Gartner logo on my blog posts ;), and you can use the report to sway your CIO.

Andrew starts by listing two misconceptions…

  • That we need stretched VLANs for VM migration (check);
  • That we need stretched VLANs for active-active data centers (check).

… and lists four things to consider:

  • WAN latency in LAN environment (check);
  • L4-7 service architecture and resulting traffic trombones (check);
  • Resiliency and split-brain (check);
  • Traffic symmetry (check);

Finally, he concludes that “While these approaches may appear as a simple fix, they often increase complexity, reduce performance and availability, and increase the overall cost of your network.” Yep.

Why Am I Writing about This?

While it’s nice to see someone else coming to the same conclusions (particularly against strong headwind of vendor marketing), what matters most is that “Gartner Said So”.

I’ve heard sad stories from dozens of engineers telling me how they tried (and failed) to use my arguments to persuade their bosses that we shouldn’t overcomplicate the network to solve other people’s problems – now they can point to a Gartner report saying exactly the same thing.

Unfortunately, for some of those engineers the report came years too late – at least a few (that I know of) already got badly burned by a meltdown spreading across multiple data centers.

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  1. Andrew has written an excellent report.

    I keep coming across L2 extension requirements quite often. In addition to reasoning with the folks who want to do this, I keep referring folks to your numerous posts on this subject. Now I am beginning to refer to Andrew's report as well :)

    In my experience, I see stretched VLAN requirements coming mostly from our European clients who want to build active/active DCs within a metro area. I see this less from clients in other geographies. Could it be the availability of cheaper dark fiber / DWDM links within metro areas in Europe?
    1. Don't think so. Must be something else. What "other geographies" are you referring to?
    2. Hi Ivan, The "other geographies" are North America and APAC. It is probably coincidence that I see this more often from our European clients :)
  2. Thank you for this. Like many we've been saying the same for years but the only way to even bring up not building L2 stretch further is the "Gartner said so" we have been so desperately waiting for!
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