Beware of Unused NICs

When you're configuring a newserver that has multiple NICs, it'simportant that you disable any NICsthat aren't plugged in to the network.If you don't disable an unused NIC,the Windows software will assign it anIP address from the 169.254.x.x subnet. This address isn't used anywhereon the network and isn't routableacross any WAN routers.

At this point you might bethinking, "So if it isn't connected, what's the problem?" Problems canarise on domain controllers (DCs)running DNS. Servers register allactive IP addresses with the defaultDNS server. On a DC, this has theside effect of registering the serverin Active Directory (AD) as a DCwith two IP addresses: its valid IPaddress and the invalid 169.254.x.xaddress.

When a client makes a DNSrequest to find all the DCs for theappropriate domain, occasionally theclient will be given the invalid 169.254.x.x address as a valid DC addressbecause addresses are returned in around-robin fashion.The client will thenattempt to contact theDC using this invalidaddress. Of course, it won't be able tocontact the DC, andthe connection will fail. The client willthen attempt to lookup another DC usingDNS. Eventually theclient will succeed, andall will be fine. However, all these extra lookups will slow downthe client computer.

We learned aboutthis problem whileattempting to connecta new storage appliance. The appliancewas able to registerwith the domain butperiodically would beunavailable when weattempted to browse for files locatedon the appliance. We had to manuallygo through DNS and remove all ofthe invalid entries and reregister theappliance with the domain to correctthe problem.

So, the moral of the story is this:If you have multiple NICs in a server,disable those NICs that aren't beingused.
—Chris Lamb, Director, IT Infrastructure, HIT Entertainment