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VMWare Question

Why do the source host and destination host need to be on the same subnet/vlan (L2 adj) for vMotion to work?

14 REPLIES
VIP Super Bronze

Re: VMWare Question

Victor,

I am no expert when it comes to VMware, but from what I have a heard from the server guys is that in the order for the Vmotion to be able to move from one cluster to another to provide redundancy the subnet has to be the same. Another word this is similar to VRRP and HSRP in the network work and in order for VRRP or HSRP to work properly between 2 routers the subnet has to be the same.

HTH

Reza 

Hall of Fame Super Blue

Re: VMWare Question

Victor

When a VM moves from one machine to another the machines cannot have different IP addresses because the IP does not change. So they must all be in the same vlan to ensure that the IP does not change.

Jon

Blue

Re: VMWare Question

Reza, I know you are right, I am asking why.

Jon, I guess you do make a good point -- that if you dont want to break the Application, the IP address of the server must be the same, lest the clients lose connectivity to the app server.

OK, but what if I want to migrate the VM and I dont care if there is a different IP address at the destination because, say, I have down time on the app anyway and it is to remain unreachable while maintenance is going on. Just a hypothetical...

There is a technical requirement with regard to the vmkernel and vCenter, I believe.

Hall of Fame Super Blue

Re: VMWare Question

Victor

Jon, I guess you do make a good point --  does it hurt that much to say it

OK, but what if I want to migrate the VM and I dont care if there is a different IP address at the destination because, say, I have down time on the app anyway and it is to remain unreachable while maintenance is going on. Just a hypothetical...

Vmotion expects and requires the servers to be in the same layer 2 broadcast domain ie. vlan. So obviously the code has been written based on that assumption.

If you want to change the IP and don't care about downtime VMotion is not the right tool for the job.

Jon

Blue

Re: VMWare Question

Jon, how have you been, buddy? Long time....hope all is well.

Jon, believe me, I am totally feeling what youre saying..Im with you...I want to know the specifics of what in the code forces this requirement...what dataflow breaks if an L2 adjacency doesnt exist?

I think the answer is that the vmkernel interfaces in each of the hypervisors view the network as a large L2 broadcast domain and base their discovery and reachability on that. I know that through vCenter, the vmKernel communicates with other vmkernels in the domain, especially during a vMotion event. Or something like that....

Blue

Re: VMWare Question

Scared you away?

Hall of Fame Super Blue

Re: VMWare Question

lamav wrote:

Jon, how have you been, buddy? Long time....hope all is well.

Jon, believe me, I am totally feeling what youre saying..Im with you...I want to know the specifics of what in the code forces this requirement...what dataflow breaks if an L2 adjacency doesnt exist?

I think the answer is that the vmkernel interfaces in each of the hypervisors view the network as a large L2 broadcast domain and base their discovery and reachability on that. I know that through vCenter, the vmKernel communicates with other vmkernels in the domain, especially during a vMotion event. Or something like that....

Victor

Funny you should ask. Had a "slight" accident on my mountain bike and fractured my spine so i'm now looking at 6 - 8 weeks of lying in bed hoping that when it sorts itself out i still have the same mobility as before. Should do but i really want to be able to carry on biking. Apart from that everything else is fine. Not doing as much NetPro recently although that may change as i have very little else i can do.

I don't know the specifics in the code but obviously most devices such as servers/pcs/printers etc. all have enough code to understand L3 concepts ie. compare destination IP with my own IP using subnet mask to see if destination is on same or different network etc..  I suspect this code is not there in Vmotion because it doesn't need to be ie. you don't add code to an application that isn't needed. VMotion doesn't care about L3 addressing as such, it is only really cares about L2 addressing. It's a bit like asking why does STP not understand about IPs, the answer being that it doesn't need to because of the network layer it works at.

I appreciate that might not answer your question fully but i don't know the exact implementation details of VMotion.

Jon

Blue

Re: VMWare Question

Fractured your spine??? What the....??? Wow, kid, take it easy! We're not 18 years old anymore, buddy...our bones crack a lot easier. lol...

I am glad you are recovering and -- God willing -- you will be back as you were before. Just follow doctor's orders and take it easy.

I mean, without you around, who the hell is gonna answer my annoying questions???

Hall of Fame Super Blue

Re: VMWare Question

Victor

Your'e right of course, we aren't getting any younger, but you still have to have some fun in life and hurtling down singletrack is mine To be honest it was a bit of a fluke landing ie. i have crashed many times before and nothing like this so it's just one of those things.

Thanks for words of support, as long as i'm doing NetPro i'll always try to answer your questions...

Jon

VIP Super Bronze

Re: VMWare Question

Jon,

I am sorry about what happened to you.  Hope you recover very soon and can get back on your bike. No wonder, I had not seen that many posts from you recently.  I thought, you are busy at work. Wish you well.  This site can not be without you.

Reza

Hall of Fame Super Blue

Re: VMWare Question

Reza

Many thanks for the kind words, much appreciated.

Jon

Cisco Employee

Re: VMWare Question

Hi Jon,

Whoa! I hope you will be back on your feet soon and all will be allright. As Reza has pointed out completely correctly, the NetPro just would not be the same without your kind guidance and help given to so many people here including myself...

Please take care.

Best regards,

Peter

Hall of Fame Super Silver

Re: VMWare Question

Hello Jon,

my best whishes for your recover.

I would have preferred to hear you were busy in a complex migration project.

On the other hand, we need also to live some time away from PCs, internet and so on. This is just human.

Best Regards

Giuseppe

Hall of Fame Super Blue

Re: VMWare Question

Hi Giuseppe

Many thanks for those kind words. As you say we do indeed need to do things other than just networking

Jon

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