Cisco WS-C3560-48PS-S Software Configuration Manual page 261

Chapter 12 Configuring VLANs
Figure 12-2 Switches in an ISL Trunking Environment
You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more
information about EtherChannel, see
Ethernet trunk interfaces support different trunking modes (see
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-Point
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.
•
•
You can also specify on DTP interfaces whether the trunk uses ISL or 802.1Q encapsulation or if the
encapsulation type is autonegotiated. The DTP supports autonegotiation of both ISL and 802.1Q trunks.
Table 12-4 Layer 2 Interface Modes
Mode
switchport mode access
switchport mode dynamic auto
78-16156-01
ISL
trunk
Switch
Switch
VLAN1
VLAN2
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 to become a trunk
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.
Function
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.
Makes the interface able to convert the link to a trunk link. The interface becomes 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.
Catalyst 6500 series
switch
ISL ISL
trunk trunk
Switch
VLAN3 VLAN2
VLAN1
Chapter 29, "Configuring EtherChannels."
Catalyst 3560 Switch Software Configuration Guide
Configuring VLAN Trunks
ISL
trunk
Switch
VLAN3
Table 12-4). You can set an interface as
12-17