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Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
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Chapter12 Configuring VLANs
Configuring VLAN Trunks
Figure12-2 Switches in an IEEE 802.1Q Trunking Environment
You can configure a trunk on a single Ethernet interface or on an EtherChannel bundl e. For more
information about EtherChannel, see Chapter 24, “Configuring EtherChannels and Layer 2 Trunk
Failover.”
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 to become a trunk
but to not generate DTP frames.
Catalyst 6000 series
switch
BladeCenter
BladeCenter
Catalyst
3500 XL
switch
Catalyst
3500 XL
switch
VLAN2
VLAN3VLAN1
VLAN1
VLAN2
VLAN3
802.1Q
trunk
802.1Q
trunk
92420
Table12-4 Layer 2 Interface Modes
Mode Function
switchport mode access Puts the interface (access port) into permanent nontrunking mode. The interface becomes
a nontrunk interface even if the neighboring interface is a trunk interface.
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. The default switch-port mode for all Ethernet interfaces is dynamic desirable.