3Com 4007 Implementation Manual page 260

260 C 14: V LAN (VLAN )
HAPTER IRTUAL S S
You can configure overlapping VLANs as long as the VLANs have some
distinguishing characteristic. For example, a bridge port can be shared
by multiple VLANs as long as there is a distinguishing characteristic for
the shared port (for example, for Multilayer Switching Modules:
protocol type or tagging type; for Layer 2 modules: tagging type). In
allClosed mode, you must tag the shared ports of any overlapped
network-based VLANs.
Consider maintaining the system's default VLAN. The default VLAN
preserves the flooding of unspecified traffic because it initially contains
all of the system's ports, with unspecified protocol information and no
tagging.
If you are using a Multilayer Switching Module to establish routing
between static VLANs and configure a VLAN interface to support one
or more routing protocols, configure the VLAN for the protocols
before you configure a routing interface. For protocols other than IP,
the module does not define the routing interface for a protocol if a
VLAN for that protocol does not exist. If you define an IP interface and
specify as the interface type, the system does not define the IP
vlan
routing interface unless you have an IP VLAN configured. See the
appropriate routing protocol chapter for an overview of your routing
options and guidelines. See Chapter 16 for information on defining
either a IP router interface for a static IP VLAN or a router port IP
interface.
If you plan to use trunks (aggregated links), define the appropriate
trunks before you define your VLANs. (If you define a VLAN with
certain ports and subsequently configure some of those ports to be
part of a trunk, the module removes those ports from the VLAN and
places them in the default VLAN.) See "Trunking and the Default
VLAN" later in this chapter for information about how trunking
actions affect the default VLAN. When you define a VLAN that
includes trunk ports, you must specify the trunk's anchor port
(lowest-numbered port). For trunking information, see Chapter 12.
When a frame is received, the frame is assigned to a VLAN using the
ingress rules. See "Ingress Rules" later in this chapter. When it
transmits the frame, the module determines the tag status (none or
IEEE 802.1Q tagging) by referring to the tag status of the transmit
port in the frame's assigned VLAN. In allOpen mode, a frame may be
transmitted on a port that does not belong to the assigned VLAN, in
which case, the frame is transmitted untagged.