Configuring Vlan Trunks; Trunking Overview - Cisco Catalyst 2975 Software Configuration Manual
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Also See for Catalyst 2975:
- Hardware installation manual (62 pages) ,
- Brochure (19 pages) ,
- Getting started manual (17 pages)
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Chapter 13
Configuring VLANs
Configuring VLAN Trunks
These sections contain this conceptual information:
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Trunking Overview
A trunk is a point-to-point link between one or more Ethernet switch interfaces and another networking device
such as a router or a switch. Ethernet trunks carry the traffic of multiple VLANs over a single link, and you
can extend the VLANs across an entire network. The switch supports IEEE 802.1Q encapsulation.
You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more
information about EtherChannel, see
Tracking."
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.
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Table 13-4
Layer 2 Interface Modes
Mode
switchport mode access
switchport mode dynamic auto
switchport mode dynamic
desirable
OL-19720-02
Trunking Overview, page 13-13
Default Layer 2 Ethernet Interface VLAN Configuration, page 13-14
Configuring an Ethernet Interface as a Trunk Port, page 13-15
Configuring Trunk Ports for Load Sharing, page 13-19
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.
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.
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.
Chapter 37, "Configuring EtherChannels and Link-State
Table
Catalyst 2975 Switch Software Configuration Guide
Configuring VLAN Trunks
13-4). You can set an interface as
13-13
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