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Chapter12 Configuring VLANs
Configuring VLAN Trunks
Ethernet trunk interfaces support different trunking modes (see Ta ble 12- 4). You can set an interface as
trunking or nontrunking or to negotiate trunking with the neighboring interface. To autonegotiate
trunking, the interfaces must be in the same VTP domain.
Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Point-to-P oint
Protocol. However, some internetworking devices might forward DTP frames improperly, which could
cause misconfigurations.
To avoid this, you should configure interfaces connected to devices that do not support DTP to not
forward DTP frames, that is, to turn off DTP.
•If you do not intend to trunk across those links, use the switchport mode access interface
configuration command to disable trunking.
•To enable trunking to a device that does not support DTP, use the switchport mode trunk and
switchport nonegotiate interface configuration commands to cause the interface t o become a trun k
but to not generate DTP frames. Use the switchport trunk encapsulation isl or switchport trunk
encapsulation dot1q interface to select the encapsulation type on the trunk port.
You can also specify on DTP interfaces whether the trunk uses ISL or IEEE 802.1Q encapsulation or if
the encapsulation type is autonegotiated. The DTP suppo rts au tonegot iati on of b ot h I SL and
IEEE 802.1Q trunks.
Note DTP is not supported on private-VLAN ports or tunnel ports.
Table12-4 Layer 2 Interface Modes
Mode Function
switchport mode access Puts the interface (access port) into permanent nontrunking mode an d negot ia tes 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.
switchport mode dynamic auto Makes the interface able to convert the link to a trunk link. The interface b ecomes a trunk
interface if the neighboring interface is set to trunk or desirable mode. The default
switchport mode for all Ethernet interfaces is dynamic auto.
switchport mode dynamic
desirable Makes the interface actively attempt to convert the link to a trunk link. The interface
becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto
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 nonegotiate Prevents the interface from generating DTP frames. You can use this command only when
the interface switchport mode is access or trunk. You must manually configure the
neighboring interface as a trunk interface to establish a trunk link.
switchport mode dot1q-tunnel Configures the interface as a tunnel (nontrunking) port to be connecte d 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 16,
“Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling,” for more infor ma tion on
tunnel ports.