Cisco Catalyst 3550 series Software Configuration Manual page 281

Chapter 12 Configuring VLANs
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.
•
•
On GigaStack GBICs, dynamic trunking is only supported when only one port of a GigaStack GBIC is
Note
being used. If trunking is required on a GigaStack GBIC where both ports are in use, use the switchport
mode trunk and switchport nonegotiate interface configuration commands on both GBIC interfaces to
cause the interfaces to become trunks.
You can also specify 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.
Tunnel ports do not support DTP. See
Note
Tunneling,"
Table 12-4 Layer 2 Interface Modes
Mode
switchport mode access
switchport mode dynamic
desirable
switchport mode dynamic auto
switchport mode trunk
switchport nonegotiate
switchport mode dot1q-tunnel
78-11194-09
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.
for more information on tunnel ports.
Function
Puts the interface (access port) into permanent nontrunking mode. The interface becomes
a nontrunk interface even if the neighboring interface is a trunk interface.
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. The default switch-port mode for all Ethernet interfaces is dynamic desirable.
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.
Puts the interface into permanent trunking mode and negotiates to convert the link into a
trunk link. The interface becomes a trunk interface even if the neighboring interface is
not a trunk interface.
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.
Configures the interface as a tunnel (nontrunking) port to be connected in an asymmetric
link with an 802.1Q trunk port. 802.1Q tunneling is used to maintain customer VLAN
integrity across a service provider network. See
Layer 2 Protocol Tunneling,"
Chapter 15, "Configuring 802.1Q and Layer 2 Protocol
Chapter 15, "Configuring 802.1Q and
for more information on tunnel ports.
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring VLAN Trunks
12-17