Cisco 7609-S Software Configuration Guide - Page 210

Understanding How Private VLANs Work Chapter 15 Configuring Private VLANs Private VLAN Port Isolation You can use private VLANs to control access to end stations in these ways: • Configure selected interfaces connected to end stations as isolated ports to prevent any communication at Layer 2. For example, if the end stations are servers, this configuration prevents Layer 2 communication between the servers. • Configure interfaces connected to default gateways and selected end stations (for example, backup servers) as promiscuous ports to allow all end stations access to a default gateway. You can extend private VLANs across multiple devices by trunking the primary, isolated, and community VLANs to other devices that support private VLANs. To maintain the security of your private VLAN configuration and to avoid other use of the VLANs configured as private VLANs, configure private VLANs on all intermediate devices, including devices that have no private VLAN ports. IP Addressing Scheme with Private VLANs When you assign a separate VLAN to each customer, an inefficient IP addressing scheme is created as follows: • Assigning a block of addresses to a customer VLAN can result in unused IP addresses. • If the number of devices in the VLAN increases, the number of assigned addresses might not be large enough to accommodate them. These problems are reduced by using private VLANs, where all members in the private VLAN share a common address space, which is allocated to the primary VLAN. Hosts are connected to secondary VLANs, and the DHCP server assigns them IP addresses from the block of addresses allocated to the primary VLAN. Subsequent IP addresses can be assigned to customer devices in different secondary VLANs, but in the same primary VLAN. When new devices are added, the DHCP server assigns them the next available address from a large pool of subnet addresses. 15-4 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX OL-4266-08