3Com 10002211 manual Overlapped IP VLANs

About VLANs C-7

Overlapped IP VLANs

The CoreBuilder system also gives you the ability to assign network layer information to IP VLANs. This capability allows network administrators to manage their VLANs by subnetwork. Flooding decisions are made by first matching the incoming frame using the protocol (IP) and then matching it with layer 3 subnetwork information. If received data is IP but does not match any defined IP subnetwork VLAN, it is flooded within all IP VLANs using the relevant switch port.

For example, two IP VLANs can be configured for ports 1-10 as follows:

IP VLAN 1 - subnet 158.101.112.0, ports 1-10

IP VLAN 2 - subnet 158.101.113.0, ports 1-10

This example shows how flooding decisions are made using overlapping IP VLANs (assuming a 12-port switch):

		Network
Index	VLAN	Address/Mask	Ports
1	Default	none	1	- 12
2	IP	158.103.122.0/	1	- 6
		255.255.255.0
3	IP	158.103.123.0/	6	- 12
		255.255.255.0

Data received on	Is flooded on	Because
IP subnet	VLAN 2	IP network layer matches layer 3 address for
158.103.122.2		VLAN 2.
on port 6
IP subnet	VLAN 3	IP network layer matches layer 3 address for
158.103.123.2		VLAN 3.
on port 6
IP subnet	VLAN 2 and	IP network layer does not match any layer 3
158.103.124.2	VLAN 3	address for IP VLANs.
on port 6
IPX on port 6	VLAN 1	IPX frame does not match any defined VLAN.

As shown in this example, when the subnet address of an IP packet does not match any subnet address of any defined IP VLAN in the system, it is flooded to all of the IP VLANs that share the source switch port, in this case, port 6.