Oct 9, 2011
tom

How Windows choose what network adapter to use when redundant adapters are available?

Question

I have a following configuration: two Windows Server 2008 non-virtual machines (a database server and an application server) are connected to the same network through two network adapters, a fast one (1 Gbps), and a slow one (100 Mbps). They have a different IP, but share the same configuration.

The schema of the network installation

The application server requests data from another server either from the database or from file shares. It connects to the shares using the machine name: \DataServer01<FileName>. The first IP associated with \DataServer01 in DNS server is 192.168.1.19 (used by 1 Gbps adapter). I want it to be used every time, and the slow one to be used only if the fast one fails.

Sometimes, the application server downloads the files from the share at the maximum speed, but sometimes, the transfer still uses the fast 192.168.1.22 at application server side, but the slow 192.168.1.18 at database server side, limiting the speed to ≈11 MB/s.

I don’t have precise metrics, but from what I’ve seen, I imagine that it fails to use the default connection half of the time, randomly.

If I specify \192.168.1.19<FileName> instead of \DataServer01<FileName>, everything works well and at maximum speed.

How to diagnose what’s happening? Is there a policy which forces Windows to choose random network adapters when sending files from a share? Are there settings to check in DNS server (a role of Windows Server 2008)?

Answer

As devicenull said, this is a problem of name resolution.

What I suspect is happening is that you are using NetBIOS name resolution. Without a WINS server running on the network, this name resolution works via network broadcast.

Whichever card on the server happens to reply to this broadcast first is the one that will get used, until the name resolution cache entry expires (10 or 15 minutes I think), and then there will be another broadcast.

You can read more about this here: http://www.techrepublic.com/article/how-netbios-name-resolution-really-works/5034239

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