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Configuring VLANs
VLANs
In a network of Cisco switches connected through IEEE 802.1Q trunks, the switches maintain one spanning-tree
instance for each VLAN allowed on the trunks. Non-Cisco devices might support one spanning-tree instance for all
VLANs.
When you connect a Cisco switch to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco switch combines
the spanning-tree instance of the VLAN of the trunk with the spanning-tree instance of the non-Cisco IEEE 802.1Q
switch. However, spanning-tree information for each VLAN is maintained by Cisco switches separated by a cloud of
non-Cisco IEEE 802.1Q switches. The non-Cisco IEEE 802.1Q cloud separating the Cisco switches is treated as a
single trunk link between the switches.
Make sure the native VLAN for an IEEE 802.1Q trunk is the same on both ends of the trunk link. If the native VLAN
on one end of the trunk is different from the native VLAN on the other end, spanning-tree loops might result.
Disabling spanning tree on the native VLAN of an IEEE 802.1Q trunk without disabling spanning tree on every VLAN
in the network can potentially cause spanning-tree loops. We recommend that you leave spanning tree enabled on
the native VLAN of an IEEE 802.1Q trunk or disable spanning tree on every VLAN in the network. Make sure your
network is loop-free before you disable spanning tree.
Default Layer 2 Ethernet Interface VLAN Settings
Ethernet Interface as a Trunk Port
Because trunk ports send and receive VTP advertisements, to use VTP you must ensure that at least one trunk port is
configured on the switch and that this trunk port is connected to the trunk port of a second switch. Otherwise, the switch
cannot receive any VTP advertisements.
Note: By default, an interface is in Layer 2 mode. The default mode for Layer 2 interfaces is switchport mode dynamic
auto. If the neighboring interface supports trunking and is configured to allow trunking, the link is a Layer 2 trunk or, if
the interface is in Layer 3 mode, it becomes a Layer 2 trunk when you enter the switchport interface configuration
command.
Trunking Interaction with Other Features
Trunking interacts with other features in these ways:
A trunk port cannot be a secure port.
A trunk port cannot be a tunnel port.
Trunk ports can be grouped into EtherChannel port groups, but all trunks in the group must have the same
configuration. When a group is first created, all ports follow the parameters set for the first port to be added to the
group. If you change the configuration of one of these parameters, the switch propagates the setting you entered to
all ports in the group:
— Allowed-VLAN list.
— STP port priority for each VLAN.
Feature Default Setting
Interface mode switchport mode dynamic auto
Allowed VLAN range VLANs 1 to 4096
VLAN range eligible for pruning VLANs 2 to 1001
Default VLAN (for access ports) VLAN 1
Native VLAN (for IEEE 802.1Q trunks) VLAN 1
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