Brocade TurboIron 24X Series Configuration Guide - Manual (Page 427)

Brocade TurboIron 24X Series Configuration Guide 393
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Configuring the same IP subnet address on multiple port-based VLANs
FIGURE 47 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
Layer 3 Switches configured for VRRP (Virtual Router Redundancy Protocol).
The 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 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 device performs a proxy ARP on behalf
of the other host. The device then replies to the ARP by sending the virtual routing interface
MAC address. The 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 MAC address, the device switches the packet on Layer 3 to the destination host on
the VLAN.
NOTE
If the device ARP table does not contain the requested host, the device forwards the ARP
request on Layer 2 to the same VLAN as the one that received the ARP request. Then the
device sends an ARP for the destination to the other VLANs that are using the same IP subnet
address.
VLAN 2
VLAN 3
VLAN 4
VLAN 2
VE 1
-IP 10.0.0.1/24 VLAN 3
VE 2
-Follow VE 1 VLAN 4
VE 3
-Follow VE 1
Switch