Neither answer is right for every case.
Ultimately for large deployments, out-of-band NAC that uses some form of edge enforcement such as access switches will be more practical because it scales better, Whiteley says. In-band NAC requires more and more devices as the rollout grows.
But for smaller networks or for targeted NAC enforcement, in-line NAC appliances can serve just as well, says Joel Snyder, a senior partner in Opus One and a member of the Network World Test Alliance. "In-band I think of more for the occasional guest access -- drop one of those boxes in between your guests and let it handle that load," Snyder said in a recent Network World online chat about NAC.
Out-of-band NAC relies on existing network mechanisms to enforce policies, such as 802.1X authentication, ARP and MAC tables and DHCP assignment. It gets its name from the fact that the NAC device does not sit directly in the flow of traffic - usually it hangs off a switch port - without any direct means to restrict traffic.
In-band NAC includes devices that traffic must pass through, usually placed between access-layer devices and distribution layer devices, although some can sit between distribution and core layers of networks. In some cases, vendors have incorporated NAC capabilities in access switches themselves.
There are many arguments in favor of both methods. Network World asked two vendors -- one selling inline gear, one selling out-of-band -- to come up with the best reasons for their positions.
Quick deployment without interrupting business use of the network, no single point of failure and lower risk of creating network latency are cited as advantages of out-of-band deployment by Grant Hartine, CTO of NAC vendor Mirage Networks.
He also says out-of-band NAC gear finds devices when they send ARP packets that may be missed by in-line devices, depending on their placement.
By contrast, in-band NAC provides better control of traffic and can monitor and restrict traffic after devices are admitted to networks if they violate usage policies, says Jeff Prince, CTO of NAC vendor ConSentry Networks. In-band deployment also supports enforcement of user-specific policy rules about what resources a person can access without creating extensive virtual LANs to address each case, he says.
"Out -of-band is what I like to call 'edge enforcement," Snyder says. "It scales, it handles the load and it doesn't depend on a single point to do enforcement. [In-line] is really where I think we want to go for big enterprise deployments.
"Of course, that doesn't mean that the in-band guys can't handle the load, but you really want to aim for edge enforcement if it fits, and go for in-band if it doesn't."