Multipath for iSCSI storage using one or two vlans and switches

Posted on 2012-03-29
Medium Priority
Last Modified: 2012-05-07
I'm setting up a HP MSA P2000 SAN that has two controllers in it.  I understand why you'd want each controller to plug into a different physcial switch for redundancy, but do you really need to have two vlans/subnets, one for each controller to connect to?  Or can you just have one and do it all through a single vlan/subnet?
Question by:jpletcher1
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Expert Comment

ID: 37791076
As you wrote, the idea of two controllers is so you can connect each one to a different switch.  I'm not sure exactly by what you mean by "Or can you just have one".  Do you mean one vlan or one switch.  If you only want to have a single switch, that's fine.  If one controller fails, the other will continue.  Each controller is usually in it's own broadcast domain, so yes I'd split the switch into two vlans, one for each controller.  Just keep in mind that if the switch fails, everything will come to a crashing halt.  You can start with this setup and in the future just add the 2nd switch with no need to bring the setup down.  In the setups I manage, I have single SAN's to single switches and multiple SAN's to multiple switches.

Author Comment

ID: 37801963
In my case, I have two physical switches, and each controller connects to a different one for redundancy.  But we have one vlan for all storage traffic.  My question is, is there really a need to have two vlans?  Or is there a reason you wouldn't want both controllers and sans all working off a single vlan network?

Expert Comment

ID: 37807552
In your case you don't need two vlans.  Treat each controller as it's own network.  On the ESX hosts, you should setup iSCSI for both controllers.  Just put the controllers on separate subnets from each other.  After your done, your iSCSI setup should have both targets.
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Expert Comment

ID: 37807814
-->In your case you don't need two vlans.
-->Just put the controllers on separate subnets from each other

From a networking standpoint typically each IP subnet is in its own VLAN.

Author Comment

ID: 37811402
Yes, I was just about to ask what giltjr mentioned.  You say I can have one vlan, but then to put my controllers on different IP subnets.  Typically a vlan is correlated to a subnet.

What is the issue with having both controllers on the same subnet and same vlan as long as they are plugged into different physical switches?

Author Comment

ID: 37936930
I'm just going to assume there are no issues with having both controllers use the same vlan and same IP subnet.  The info above is kind of contradicting in saying I don't need two vlans but I need to have separate subnets.  

Can anyone clear that up before I close this?

Accepted Solution

Fizicist earned 2000 total points
ID: 37938566
You really don't need separate subnets.  At one customer I have an Equallogic SAN which has 2 controllers in a Active/Passive layout.  So I only have to configure the IP's for one controller and they get mirrored to the 2nd when it goes active (This setup, two controllers, one subnet).  I also have one customer with an EMC san, which has 2 controllers in an Active/Active layout.  ESX multipathing keeps track of the different paths for the systems.

Look at
Page 4, two switches two servers as an example.  Not all possible setups are outlined in the PDF, but it should give you an idea.

Separate subnets may help in diagnosing a failure.  I don't have an HP San to review to check if they run on an Active/Passive setup.

Author Comment

ID: 37938588
I guess in my mind it wouldn't matter which vendor you have or if it is active/active or active/passive would it?  It seems like it would be a general principal applied to storage networking.  

I guess having separate subnets might be good in the case of if you have two subnets that each run on completely different hardware.  

In any event, thanks for the info.  I'll go ahead and close this out.

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