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Chapter 11 Configuring VLANs Configuring VLAN Trunks
You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more
information about EtherChannels, see Chapter 34, “Configuring EtherChannels and Link-State
Tracking.”
Ethernet interfaces support different trunking modes (see Table 11-4). You can set an interface as
trunking or nontrunking.
• If you do not intend to trunk across links, use the switchport mode access interface configuration
command to disable trunking.
• To enable trunking, use the switchport mode trunk interface configuration command to change the
interface to a trunk.
IEEE 802.1Q Configuration Considerations
The IEEE 802.1Q trunks impose these limitations on the trunking strategy for a network:
• 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 that 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.
Tab le 11-4 Layer 2 Interface Modes
Mode Function
switchport mode access Puts the interface (access port) into permanent nontrunking mode and negotiates to
convert the link into a nontrunk link. The interface becomes a nontrunk interface
regardless of whether or not the neighboring interface is a trunk interface. This is the
default mode.
switchport mode trunk Puts the interface into permanent trunking mode and negotiates to convert the
neighboring link into a trunk link. The interface becomes a trunk interface even if the
neighboring interface is not a trunk interface.
switchport mode dot1q-tunnel Configures the interface as a tunnel (nontrunking) port to be connected in an asymmetric
link with an IEEE 802.1Q trunk port. The IEEE 802.1Q tunneling is used to maintain
customer VLAN integrity across a service provider network. See Chapter 13,
“Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling,” for more information on
tunnel ports.
switchport mode private-vlan Configure the interface as a private VLAN host or promiscuous port (only NNIs can be
configured as promiscuous ports). For information about private VLANs, see Chapter 12,
“Configuring Private VLANs.”