For IP VLANs only, you can also configure network-layer subnetwork addresses. With this additional Layer 3 information, you can create multiple independent IP VLANs with the same bridge ports. Untagged frames are assigned to a network-based VLAN according to both the protocol (IP) and the Layer 3 information in the IP header. Assigning
Layer 3 address information to IP VLANs allows network administrators to manage their IP routing interfaces by subnetwork.

Network-based IP VLANs accommodate multiple routing interfaces over the same set of ports without tagging. Therefore, this option can be useful in allOpen mode. In allClosed mode, overlapped network-based IP VLANs must be IEEE 802.1Q tagged, which means that the system does not use the Layer 3 information.

Important Considerations

When you create this type of VLAN interface, review these guidelines:

• The network information is used only in situations where there are multiple network-based VLANs defined on a particular port. In situations where there is only one network-based VLAN defined on a port, the VLAN is treated as an ordinary IP protocol-based VLAN, and network-based information is ignored.

• When they are overlapped, network-based VLAN interfaces take precedence over protocol-based and port-based VLAN interfaces.

• You can define only one IP routing interface for a network-based VLAN. When you define an IP routing interface with the interface type vlan, the system will not allow you to select a network-based IP VLAN that already has a routing interface defined for it. For more information on IP routing interfaces, see Chapter 16.

• If you define multiple interfaces for an IP VLAN (instead of defining a network-based VLAN), you cannot subsequently modify that IP VLAN to supply Layer 3 address information. If only one routing interface is defined for the IP VLAN, then you can supply Layer 3 address information as long as it matches the Layer 3 information specified for the routing interface.

• In allClosed VLAN mode, you must also supply IEEE 802.1Q tagging for the ports (overlapped). Therefore, this feature has no added benefit. After IEEE 802.1Q tagging is implemented, implicit VLAN membership information such as the protocol or Layer 3 IP network address is not used, and the frame is assigned to the VLAN based solely on the tag VID and the receive port.

• In allOpen mode, you need not supply the IEEE 802.1Q tagging. However, to ensure line-speed throughput for overlapped network-based IP VLANs in allOpen mode, supply the IEEE 802.1Q tagging.

Figure 21 shows two IP network-based VLANs and two IPX protocol-based VLANs. (The switch fabric module resides in slot 8 but is logically represented above the other modules.)

In this configuration:

• There are two Multilayer Switching Modules and the switch fabric module. One module can handle servers; the other module can handle clients.

• There are four user-configured VLANs and the default VLAN:

  • VLAN1, the default VLAN on all modules, is untagged. The default VLAN is not represented in the figure.

  • VLAN2, a network-based VLAN for IP with Layer 3 address 22.2.2.0 on the Multilayer Switching Module in slot 3. The IP routing interface for IP VLAN 2 (22.2.2.10) on the Multilayer Switching Module in slot 3 is on the same subnet as the IP routing interface that is defined for VLAN2 on the Multilayer Switching Module in slot 5 (22.2.2.20).

  • VLAN3, a protocol-based VLAN for IPX-802.3 on the Multilayer Switching Module in slot 3. The IPX routing interface is defined to be on the same IPX network (1) as the IPX routing interface that is defined for VLAN3 on the Multilayer Switching Module in slot 5. Although there are overlapping ports in VLAN2 and VLAN3, the protocol type is the distinguishing factor.

  • VLAN4, a network-based VLAN for IP with Layer 3 address 44.4.4.0 on the Multilayer Switching Module in slot 5. The IP routing interface for IP VLAN 4 is 44.4.4.10.

  • VLAN5, a protocol-based VLAN for IPX-802.3 on the Multilayer Switching Module in slot 5. The IPX routing interface is defined to be on another IPX network (2).

Figure 21 Network-based VLANS

Table 56 defines the VLANs in this configuration:

Table 56 Network-based IP VLANs and IPX VLANs

Slot 3 Module	Slot 5 Module	Switch Fabric Module
VLAN2: VLAN Index 2, VID 20 Ports 1-8, 13 Protocol type IP 22.2.2.0 Layer 3 address Tagging none front-panel ports Tagging 802.1Q backplane port 13 IP router interface: 22.2.2.10	VLAN2: VLAN Index 2, VID 20 Port 13 Protocol type IP 22.2.2.0 Layer 3 address Tagging 802.1Q backplane port 13 IP router interface: 22.2.2.20	VLAN2: VLAN Index 2, VID 20 Ports 9,17 Tagging 802.1Q fabric ports 9,17
VLAN3: VLAN Index 3, VID 30 Ports 6-12,13 Protocol type IPX- 802.3 Tagging none ports 6-12 Tagging 802.1Q backplane port 13 IPX routing interface defined for IPX network 1	VLAN3: VLAN Index 3, VID 30 Port 13 Protocol type IPX-802.3 Tagging 802.1Q backplane port 13 Router interface defined for same IPX network 1	VLAN3: VLAN Index 3, VID 30 Ports 9,17 Tagging 802.1Q fabric ports 5,9
-	VLAN4: VLAN Index 4, VID 40 Ports 1-6 Protocol type IP 44.4.0 Layer 3 address Tagging none for front-panel ports IP router interface: 44.4.4.10	-
-	VLAN5: VLAN Index 5, VID 50 Ports 8-12 Protocol type IPX-802.3 Tagging none for front-panel ports IPX router interface defined for IPX network 2	-