Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 55)

Brocade Virtual ADX Switch and Router Guide 41
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Configuring the same IP subnet address on multiple port-based VLANs
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FIGURE 11 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’s Layer 2
broadcasts.
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 routing
interface’s MAC address, the device switches the packet on Layer 3 to the destination host on
the VLAN.