3Com 4007, Implementation Manual, Page: 289 / 684

Network-based IP VLANs 289
Network-based IP
VLANs
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.