C
HAPTER
36
| VLAN Commands
Configuring VLAN Interfaces
– 844 –
COMMAND USAGE
◆ Use this command to configure a tunnel across one or more
intermediate switches which pass traffic for VLAN groups to which they
do not belong.
The following figure shows VLANs 1 and 2 configured on switches A and
B, with VLAN trunking being used to pass traffic for these VLAN groups
across switches C, D and E.
Without VLAN trunking, you would have to configure VLANs 1 and 2 on
all intermediate switches – C, D and E; otherwise these switches would
drop any frames with unknown VLAN group tags. However, by enabling
VLAN trunking on the intermediate switch ports along the path
connecting VLANs 1 and 2, you only need to create these VLAN groups
in switches A and B. Switches C, D and E automatically allow frames
with VLAN group tags 1 and 2 (groups that are unknown to those
switches) to pass through their VLAN trunking ports.
◆ The following restrictions apply to this feature:
■
VLAN trunking can only be enabled on Gigabit Ethernet ports or
trunks.
■
VLAN trunking is mutually exclusive with the “access” switchport
mode (see the switchport mode command). If VLAN trunking is
enabled on an interface, then that interface cannot be set to access
mode, and vice versa.
◆ To prevent loops from forming in the spanning tree, all unknown VLANs
will be bound to a single instance (either STP/RSTP or an MSTP
instance, depending on the selected STA mode).
◆ If both VLAN trunking and ingress filtering are disabled on an interface,
packets with unknown VLAN tags will still be allowed to enter this
interface and will be flooded to all other ports where VLAN trunking is
enabled. (In other words, VLAN trunking will still be effectively enabled
for the unknown VLAN).
EXAMPLE
The following example enables VLAN trunking on ports 27 and 28 to
establish a path across the switch for unknown VLAN groups:
Console(config)#interface ethernet 1/27
Console(config-if)#vlan-trunking
Console(config-if)#interface ethernet 1/28
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