Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 392)

FIGURE 86 Multiple port-based VLANs with the same protocol address
Each VLAN still requires a separate virtual routing interface. However, all three VLANs now use the
same IP subnet address.
In addition to conserving IP subnet addresses, this feature allows containment of Layer 2 broadcasts
to segments within an IP subnet. For ISP environments where the same IP subnet is allocated to
different customers, placing each customer in a separate VLAN allows all customers to share the IP
subnet address, while at the same time isolating them from one another Layer 2 broadcasts.
NOTE
You can provide redundancy to an IP subnet address that contains multiple VLANs using a pair of
Brocade Layer 3 Switches configured for Brocade VRRP (Virtual Router Redundancy Protocol).
The Brocade device performs proxy Address Resolution Protocol (ARP) for hosts that want to send IP
traffic to hosts in other VLANs that are sharing the same IP subnet address. If the source and
destination hosts are in the same VLAN, the Brocade device does not need to use ARP:
• If a host attached to one VLAN sends an ARP message for the MAC address of a host in one of the
other VLANs using the same IP subnet address, the Brocade device performs a proxy ARP on
behalf of the other host. The Brocade device then replies to the ARP by sending the virtual routing
interface MAC address. The Brocade device uses the same MAC address for all virtual routing
interfaces.When the host that sent the ARP then sends a unicast packet addressed to the virtual
VLANs
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