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Catalyst 3750 Switch Command Reference
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Chapter 2 Catalyst 3750 Switch Cisco IOS Commands
switchport mode
When you enter dynamic auto mode, the interface converts the link to a trunk link if the neighboring
interface is set to trunk or desirable mode.
When you enter dynamic desirable mode, the interface becomes a trunk interface if the neighboring
interface is set to trunk, desirable, or auto mode.
To autonegotiate trunking, the interfaces must be in the same VLAN Trunking Protocol (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, which turns 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 to become a trunk
but to not generate DTP frames.
When you enter dot1q-tunnel, the port is set unconditionally as an IEEE 802.1Q tunnel port.
Access ports, trunk ports, and tunnel ports are mutually exclusive.
Any IEEE 802.1Q encapsulated IP packets received on a tunnel port can be filtered by MAC access
control lists (ACLs), but not by IP ACLs. This is because the switch does not recognize the protocol
inside the IEEE 802.1Q header. This restriction applies to router ACLs, port ACLs, and VLAN maps.
Configuring a port as an IEEE 802.1Q tunnel port has these limitations:
• IP routing and fallback bridging are not supported on tunnel ports.
• Tunnel ports do not support IP ACLs.
• If an IP ACL is applied to a trunk port in a VLAN that includes tunnel ports, or if a VLAN map is
applied to a VLAN that includes tunnel ports, packets received from the tunnel port are treated as
non-IP packets and are filtered with MAC access lists.
• Layer 3 quality of service (QoS) ACLs and other QoS features related to Layer 3 information are
not supported on tunnel ports.
For more information about configuring IEEE 802.1Q tunnel ports, see the software configuration guide
for this release.
The IEEE 802.1x feature interacts with switchport modes in these ways:
• If you try to enable IEEE 802.1x on a trunk port, an error message appears, and IEEE 802.1x is not
enabled. If you try to change the mode of an IEEE 802.1x-enabled port to trunk, the port mode is
not changed.
• If you try to enable IEEE 802.1x on a port set to dynamic auto or dynamic desirable, an error
message appears, and IEEE 802.1x is not enabled. If you try to change the mode of an
IEEE
802.1x-enabled port to dynamic auto or dynamic desirable, the port mode is not changed.
• If you try to enable IEEE 802.1x on a dynamic-access (VLAN Query Protocol [VQP]) port, an error
message appears, and IEEE 802.1x is not enabled. If you try to change an IEEE 802.1x-enabled port
to dynamic VLAN assignment, an error message appears, and the VLAN configuration is not
changed.
Examples This example shows how to configure a port for access mode:
Switch(config)# interface gigabitethernet2/0/1
Switch(config-if)# switchport mode access
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