Dual NIC's on the same subnet

Hi Experts,

Hoping you can clarify something. One of our clients requested a VM with multiple NIC's on the same subnet. They are now reporting they are unable to connect to it from a different subnet on the additional IP addresses. They can however on the original IP that is assigned to the NIC that has the default gateway.

Now to me this is correct, as packets are received on the additional NIC's, but since they come from a different subnet, it will not know where to route them. As such it passes the returned packets to the NIC with the default gateway and back to the originators IP. However when the originator receives these packets, they are now from a different IP address than it was expecting and thus rejects them

Is there any way around this?
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Graham HirstIT EngineerAsked:
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oBdACommented:
Had the same problem on an IIS on W2k8 (while the same construction was working fine on W2k3); what worked was to simply add the default gateway on the additional NICs (ignoring the warnings).
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Graham HirstIT EngineerAuthor Commented:
Was it the same default gateway? And its caused no detrimental effects?
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oBdACommented:
Yes, the same gateway as for the other NIC(s) in the same network. No ill effects that I could notice.
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tankergoblinCommented:
How about your subnet??  same gateway with same subnet?
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Graham HirstIT EngineerAuthor Commented:
Hi Tankergolin,

Yup same subnet on all of them

oBdA,

Cheers mate, i'll suggest it to the client and see how they get on
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Rob WilliamsCommented:
There is little or no advantage of having multiple IPs on the same subnet and it will cause DNS issues.  Why would they want this?  If you want to improve performance and/or increase bandwidth using multiple NICs you would normally use NIC Teaming, where both NICs share 1 IP.
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Graham HirstIT EngineerAuthor Commented:
Hi Rob,

We don't, the customer does. I believe they are hosting multiple web sites on the same box and are binding by IP rather than port or URL. I have warned them of the potential issues it may cause
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Adam RayCommented:
Why not just assign multiple IPs to one NIC?

Netsh Int IPv4 Add Address <Interface Name> <IP Address> SkipAsSource=True

(The SkipAsSource works in Server 2008+ and tells it not to attempt to register that IP in DNS)

Since you're dealing with a VM you're unlikely to need multiple NICs for bandwidth purposes (if they are on the same vSwitch on the host anyway.)

As an aside, I have had various connectivity issues on workstations with multiple NICs (each with a default GW set) on the same subnet.
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tankergoblinCommented:
HI Rob, Multiple IP is useful for redundancy where if one NIC fail you can connect to the machine with another NIC . This the case where Teaming is not available.
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Graham HirstIT EngineerAuthor Commented:
Cheers for all the info guys,

The customer has reported the multiple gateways has fixed the issue in question. I've also mentioned assigning multiple IPs to the same NIC which they may try in future

Thanks again for all the help
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Rob WilliamsCommented:
I am not a lot of help here, rather a devils advocate;
-Microsoft does not support multiple default gateways and has many articles to that effect, such as; "Multiple Default Gateways Can Cause Connectivity Problems".  Keep in mind a default gateway is just that, a default when no other route exists, so how can there be 2 defaults.  If used, it is for failover which doesn't work properly in a Windows environment and does cause loss in connectivity on the first adapter.

-When you use multiple NIC's you have to pay particular attention to DNS.  Which IP is registered in DNS?  You can't have 2 registrations in the same subnet for the same hardware.

-Though multi-homing is better in that you have a single Gateway but you can still have DNS issues and as per Microsoft articles potential active directory failures.

You may find you have connectivity issues down the road.
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