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248 C
HAPTER
14: V
IRTUAL
LAN
S
(VLAN
S
)
■
For backplane ports and switch fabric module ports, you must use
tagging when these ports are shared by multiple VLANs. (Only
one
VLAN’s backplane ports can be untagged; in all other VLANs defined
across the backplane, the backplane ports must be tagged.) If you tag
the backplane port of a switching module for a VLAN, you must also
tag the corresponding switch fabric module port in that VLAN.
Devices (end stations, routers, switches, and so forth) that are connected
to an explicitly tagged front-panel port must be capable of supporting
802.1Q tagging. If the front-panel port is untagged in the VLAN to which
they belong, however, they do not have to support 802.1Q tagging.
VLAN IDs Each VLAN is identified by its VLAN ID (VID). For VLANs that you create,
the system keeps track of its used VLAN ID numbers to help you select
the next available VLAN ID. Data frames sent by the system are tagged
per IEEE 802.1Q (which contains the VID) if tagging is enabled on the
transmit port for that VLAN. Tagged IEEE 802.1Q data frames that are
received on the system are assigned to the VLAN that corresponds to
both the VID contained in the tag and the protocol type.
Be aware of these additional guidelines:
■
The default VLAN always uses the reserved VID of 1.
■
Before assigning a VID, review the information in Table 49.
On Multilayer Switching Modules, if you rely on dynamic configuration to
create a port-based VLAN based on GVRP updates, the VID is the unique
IEEE 802.1Q VID.
If you define a router port IP interface, the system automatically creates a
router port IP VLAN and assigns it the next available VID. See
Chapter 16
for information on router port IP interfaces.
Table 49 Assigning ID Numbers to VLANs
VLAN ID
Number
Description
VID 1 Reserved for the default VLAN assigned by IEEE and
3Com Corporation
VID 4095 Reserved
VID 2–4094 Numbers that you assign when you create VLANs