To configure a private VLAN, the switch must be in Virtual Terminal Protocol (VTP) transparent mode.

A network vulnerability of private VLANs involves the use of a proxy to bypass access restrictions of the private VLAN. In a proxy attack, frames are forwarded to a host on the network connected to a promiscuous port, such as a router. The network attacker sends a packet with its source IP and MAC address and a destination IP address of the target system but a destination MAC address of the router. The switch forwards the frame to the router. The router routes the traffic, rewrites the destination MAC address as that of the target, and sends the packet out. Because the router is authorized to communicate with the private VLANs, the packet is forwarded to the target system. This type of attack allows for unidirectional traffic only because the private VLAN filter blocks the target's attempts to respond. This vulnerability is not a private VLAN vulnerability per se because all the rules of that VLAN were enforced.

Figure 14-6 shows how the private VLAN proxy vulnerability works.

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