Enterasys Networks 9034385 - Manual (Page 93)

Inline NAC Design Procedures
Enterasys NAC Design Guide 5-29
However, the closer the NAC Controller is placed to the edge of the network, the more NAC
Controllers are required on the network, increasing NAC deployment cost and complexity.
Conversely, when moving the NAC Controller towards the core of the network, fewer NAC
Controllers are required, decreasing NAC deployment cost and complexity, but also decreasing
the level of security.
For implementing NAC on wired and wireless LANs, it is recommended that the Layer 2 NAC
Controller is positioned between the access layer and distribution layer before the first routed hop
in the network. As an alternative, the NAC Controller may be positioned deeper into the network
after the first routed hop using the Layer 3 configuration. The Layer 3 NAC Controller can also be
positioned after a VPN concentrator or WAN connection to implement NAC for remote users.
Unlike the out‐of‐band NAC design, the implementation of remediation and/or MAC (network)
registration does not affect the location of the NAC Controller. The NAC Controller will
appropriately intercept web traffic for the purpose of remediation and registration.
Lastly, it should be understood that some advantages exist with the deployment of a Layer 2 NAC
Controller over a Layer 3 NAC Controller, which may affect the decision of how NAC Controllers
are positioned. While a Layer 2 NAC Controller always knows the MAC address of the
downstream connected end‐system, the Layer 3 NAC Controller may not be able to determine the
MAC address of a downstream end‐system (denoted as “Unknown” in NAC Manager.)
Techniques such as NetBIOS lookups and DHCP snooping are implemented to attempt to resolve
the IP address of the downstream connected end‐systems; however, scenarios exist where the IP
address of the downstream end‐system may not be determined.
The MAC address of a downstream end‐system will be determined by the NAC Controller in the
following scenarios:
• End‐systems support NetBIOS and a host firewall does not drop inbound NetBIOS requests
for the LAN connection.
• DHCP is implemented and the DHCP server exists upstream from the NAC Controller.
Since the Layer 3 NAC Controller may not be able to determine the MAC address of a
downstream end‐system, “Lock MAC” and MAC overrides are not applicable to Layer 3 NAC
Controllers. Furthermore, MAC (network) registration may not be implemented when the MAC
address of a downstream connected end‐system is unknown. In this case, the end‐system is
assigned the Security Domain’s default NAC configuration.