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Should a Metropolitan Area Network (MAN) be setup with a router at each site? Or, because the connections between sites are usually at layer two, should each site operate on one "master" switch which then interconnects them through a router at the main site. Or are both setups viable options?

The specific scenario I am interested is setting up a MAN for about 5 sites, each with only about 50 users and each of the sites will be interconnected through one "main" site which will also house all the servers.

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Yes. You do not want to extend Layer 2 MAN links beyond the border of each location - this is bad design for a number of reasons:

  • You will extend the broadcast domain of all 5 sites up to the main site
  • Layer 2 issues (STP re-convergence, broadcast storms etc) can now affect multiple sites at once
  • It will not scale in the future as you bring on more sites, or have an outlying site that needs to come in over a WAN link

Now whether you use an actual router, or a layer 3 switch with a routing protocol is up to you (switches are generally a lot better price/performance for basic L3 routing, at the cost of scale and features), but the result is the same.

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  • Thanks, that makes a lot of sense. I hadn't considered the broadcast domain and was getting myself wound up over the limited MAN documentation.
    – Jason
    Mar 29, 2018 at 11:51
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    yeah - MAN doesn't really describe a technology or an architecture, just a coverage area - searching Metro-Ethernet is probably where you'll find the most information, but most of that info is from the carrier's perspective, and very much L2 everywhere
    – Benjamin Dale
    Mar 29, 2018 at 14:14

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