3Com corebuilder 3500 Implementation Manual page 279

Important Considerations
Be aware of the following points when you use port-based routing:
Your system can be in only allOpen or allClosed VLAN mode. You
cannot create mixed VLAN modes on the same device.
You can establish up to 32 IP interfaces on a single VLAN.
If you set up your IP router as a port-based router, you are not required
to use 802.1Q tagging. The default VLAN that the system creates for a
port-based router interface does not enable tagging by default.
If you set up your IP router as a port-based router, you determine
whether or not you want unknown destination packets flooding that
port. To prevent the possibility of flooding unknown destination
packets onto that port, you must manually remove the port-based
router interface from the default VLAN.
A port can either be part of a VLAN interface or part of a router port
interface, but never both.
Once you have configured a port to be part of a protocol VLAN, you
cannot use it to create a router port interface of the same protocol,
and vice versa. However, if a port is part of an IPX VLAN, then you can
use that port to create an IP router port interface.
In order to support router port IP interfaces, the system must operate
in allClosed VLAN mode:
To prevent MAC addresses from being shared between other
VLANs and the router port
To ignore Spanning Tree states on the router port
This means you can configure loops among router port interfaces
and have the routing protocol make decisions rather than being
governed by the bridge.
The current default mode for VLANs is allOpen. In changing from
allOpen to allClosed mode, you must reconfigure your VLANs and
router interfaces.
allClosed mode prevents unicast traffic from being forwarded
(bridged) between VLANs.
In allClosed mode, you must define router interfaces in order to pass
traffic between VLANs. A traditional router allows the user to
configure whether or not bridging is allowed. To disable bridging on a
router port interface, the user will be required to remove that port
from the default VLAN.
Routing Models: Port-based and VLAN-based 279