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When setting up internal cloud, the recommended best practice seems to be to isolate data traffic from management traffic by using separate L2 networks. For example, see the diagram in the OpenStack Quantum documentation).

Note that this management network is used for the various cloud-related services to coordinate. It does not refer to out-of-band management using something like IPMI.

Assuming you have two switches, and two physical NICs on each compute node, this isolation is simple to implement. However, you could also configure the switches for link aggregation (aka bonding, trunking), which would protect against one of the switches failing. In this scenario, you could use VLANs to isolate the networks.

What are the tradeoffs associated with using a physical "data" switch and a "management" switch versus link aggregation + VLANs? In particular, other than the complexity of setting up the switches and the compute hosts to support link aggregation and VLANs, what are the disadvantages of that approach?

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What are the tradeoffs associated with using a physical "data" switch and a "management" switch versus link aggregation + VLANs? In particular, other than the complexity of setting up the switches and the compute hosts to support link aggregation and VLANs, what are the disadvantages of that approach?

I think your only real advantage of using physically discrete networks in this situation is conceptual simplicity. Sometimes it's just simpler to look at two physically discrete devices. I can also imagine that there are situations where you are required for legal or security reasons to have physically discrete networks. I'm thinking along the lines of three-letter agencies that require separate rooms, infrastructure and levels of clearance.

The disadvantages are multitude: It's twice as expensive, takes up twice as much space, there's twice as many pieces of hardware to fail, you trade logical complexity for physical complexity, and it's not very scalable.

Unless you have a good reason to need physically discrete infrastructure for these roles you're best off using VLANs.

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