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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.
Figure 552: Configuring VLAN Trunking
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.
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).
E
XAMPLE
The following example enables VLAN trunking on ports 9 and 10 to
establish a path across the switch for unknown VLAN groups:
Console(config)#interface ethernet 1/9
Console(config-if)#vlan-trunking
Console(config-if)#interface ethernet 1/10
Console(config-if)#vlan-trunking
Console(config-if)#
– 1351 –
| VLAN Commands
C
40
HAPTER
Configuring VLAN Interfaces
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