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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 9 Creating and Maintaining VLANs
Understanding VLAN Trunks
Note
DTP is a point-to-point protocol. However, some internetworking devices might forward DTP frames
improperly. To avoid this, ensure that interfaces connected to devices that do not support DTP are
configured with the access keyword if you do not intend to trunk across those links. To enable
trunking to a device that does not support DTP, use the nonegotiate keyword to cause the interface
to become a trunk but to not generate DTP frames.
Encapsulation Types
Table 9-7
lists the Ethernet trunk encapsulation types and keywords.
Note
The switch does not support Layer 3 trunks; you cannot configure subinterfaces or use the
encapsulation keyword on Layer 3 interfaces. The switch does support Layer 2 trunks and Layer 3
VLAN interfaces, which provide equivalent capabilities.
The trunking mode, the trunk encapsulation type, and the hardware capabilities of the two connected
interfaces determine whether a link becomes an ISL or 802.1Q trunk.
Table 9-6
Layer 2 Interface Modes
Mode
Function
switchport mode access
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 even if
the neighboring interface is not 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.
switchport mode dynamic auto
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.
switchport mode trunk
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.
switchport nonegotiate
Puts the interface into permanent trunking mode but prevents the interface from
generating DTP frames. You must manually configure the neighboring interface as a
trunk interface to establish a trunk link.
Table 9-7
Ethernet Trunk Encapsulation Types
Encapsulation
Function
switchport trunk encapsulation isl
Specifies ISL encapsulation on the trunk link.
switchport trunk encapsulation dot1q
Specifies 802.1Q encapsulation on the trunk link.
switchport trunk encapsulation negotiate
Specifies that the interface negotiate with the neighboring interface to
become an ISL (preferred) or 802.1Q trunk, depending on the configuration
and capabilities of the neighboring interface.
