Design & Build

How To: Configure Multiple VLANs with the Same IP Address

by brcd-campus.expert on ‎11-04-2011 05:33 PM (9,581 Views)

BRCD-ENTERPRISE 2307

Introduction

Classical inter-VLAN routing implies dedicating a subnet for each VLAN. If you need to conserve IP subnet addresses, you can configure multiple VLANs with the same IP subnet addresses.

Figure 1: inter-VLAN Routing Diagram

2307-1.jpg

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 Layer 2 broadcasts.

Before You Begin

Components Used
  • FastIron FCX that runs FCXR07100a.bin (Router - Top)
  • FastIron FCX that runs FCXR07100a.bin (Router IP Follow)

Topic of Discussion

Figure 2: Network Diagram
2307-2.jpg
Configuration
  • Router (IP)

!

  ip route 0.0.0.0 0.0.0.0 192.168.5.4

!

interface ethernet 1/1/5

   ip address 192.168.5.5 255.255.255.0

!

  • Router (IP Follow)

!

vlan 3 by port

   untagged ethe 1/1/3

   router-interface ve 3

!

vlan 4 by port

   untagged ethe 1/1/4

   router-interface ve 4

!

ip proxy-arp

interface ethernet 1/1/5

   ip address 192.168.5.4 255.255.255.0

!

interface ve 3

   ip address 192.168.3.1 255.255.255.0

!

interface ve 4

ip follow ve 3

!

Verify

By default, there is no inter-VLANs communication. You need to configure ip proxy-arp if you want to modify this behavior. Then, the Brocade device will perform 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.

Router_Top#show ip route

Total number of IP routes: 2, avail: 11998 (out of max 12000)

B:BGP D:Connected  R:RIP  SSmiley Frustratedtatic  OSmiley SurprisedSPF *:Candidate default

       Destination     NetMask        Gateway        Port       Cost   Type 

       0.0.0.0         0.0.0.0        192.168.5.4    1/1/5      1      S

1      192.168.5.0     255.255.255.0  0.0.0.0        1/1/5      1      D

Router_IP_Follow#show ip route

Total number of IP routes: 2, avail: 11998 (out of max 12000)

B:BGP D:Connected  R:RIP  SSmiley Frustratedtatic  OSmiley SurprisedSPF *:Candidate default  

      Destination       NetMask         Gateway         Port       Cost   Type 

1     192.168.3.0       255.255.255.0   0.0.0.0         v3         1      D

2     192.168.5.0       255.255.255.0   0.0.0.0         1/1/5      1      D


For complete configuration of the Brocade Products, visit www.brocade.com.

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