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Do More With Less: Port-Based VLANs - Page 2

 By Carla Schroder | Posted Jan 28, 2008
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What if you want west and north to communicate with each other? Put them on the same VLAN. There is one additional useful option, and that is to create master IT port, so that you as the master of the network can easily go anywhere you want on the network. Add one more port to the default that is not used by anyone else, like port 2 in this example: 

VLAN ID       Ports
01, default   1,2,16
02, east      1,3,4,16
03, west      1,5,6,7,8,16
04, north     1,9,10,11,12,16
05, south     1,13,14,15,16

Any PC connected to switch port 2 has access to the entire network. But port 2 is not a member of any other VLAN, so this keeps all control in VLAN 01.

Additional Routed Subnets

Now take a look at our next beautiful ASCII network diagram. This shows a common scenario even in very small networks- a second routed subnet, which is the wireless network segment. To make the diagram easier to read I'm using a separate wireless access point, but the WAP could also be integrated into the firewall/router: 
               broadband modem
                      |
               router/firewall
                  | |
                  | |________WAP-192.168.2.0/24
                  |
        east west default
        123 4567  8
        192.168.1.0/24

If you had routing enabled between 192.168.1.0/24 and 192.168.2.0/24 before you created your East and West VLANs, nothing has changed- all the hosts in East and West can still communicate with your wireless hosts. Because to your router, East and West are still a single subnet. This is a limitation of port-based VLANs &emdash there is no information added to the packets to identify them as belonging to a VLAN, so once they leave the switch it's as though the VLANs do not exist. So you still need to pay attention to addressing, and not have any duplicate IP addresses in your various VLANs.

Routing VLANs

Tag-based (IEEE 802.1Q) VLANs are more flexible and are routable, just like ordinary physical subnets. Next we'll learn how to set them up, how to route them, and how to configure Linux clients.

Resources

cschroder@internet.com

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