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vMotion Layer 2 Adjacency Requirement

The topic of vMotion, it’s practicality, and Layer 2 adjacency for vMotion has been a topic I’ve visited a few times over the last several months. The trend got kicked off with a post on vMotion reality, in which I attempted to debunk an article claiming vMotion was only a myth. The series continued with a discussion of the practicality of vMotion, where I again discussed the various Layer 2 requirements for vMotion.

In the vMotion practicality article, reader Paul Pindell, an employee of F5 Networks, discusses the networking requirements for vMotion. To quote from his comment:

Notice that there is no requirement for the vMotion VMkernel interfaces of the ESX(i) hosts to have what was termed Layer 2 adjacency. The vMotion VMkernel interfaces are not required to be on the same subnet, VLAN, nor on the same L2 broadcast domain. The vMotion traffic on VMkernel interfaces is routable. IP-based storage traffic on VMkernel interfaces is also routable. Thus there are no L2 adjacency requirements for vMotion to succeed.

I was intrigued by this statement, so I contacted Duncan Epping (of Yellow Bricks fame) and discussed the matter with him. Duncan has also posted on this topic on his site as well; both his post and my post are the result of our discussion and collaboration around this matter.

So is Layer 2 adjacency for vMotion a requirement, or not? In the end, the answer is that Layer 2 adjacency for VMkernel interfaces configured for vMotion is not required; vMotion over a Layer 3 interface will work. The caveat is that routed vMotion, as it has sometimes been called, hasn’t been through the extensive VMware QA process and therefore is not yet supported. (Please don’t mistake my use of the word “yet” as any sort of commitment by VMware to actually support it.)

In summary, then: vMotion across two different subnets will, in fact, work, but it’s not yet supported by VMware. As additional information becomes available—as Duncan indicated, the VMware KB article is going to be updated to avoid misunderstanding—I’ll update this post accordingly.

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