LevelOne FGL-2870 - Manual (Page 672)

Command Line Interface
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4
Command Usage
• Use this command to configure a tunnel across one or more intermdiate
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 on page 4-301). 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
Console(config-if)#vlan-trunking
Console(config-if)#