Vlan Trunks - Cisco Catalyst 4500 series Administration Manual

Chapter 18 Configuring Layer 2 Ethernet Interfaces

VLAN Trunks

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. Trunks carry the traffic of multiple VLANs over a single link and
allow you to extend VLANs across an entire network.
Only 802.1Q trunking encapsulations is available on all Catalyst 4500 Ethernet interfaces.
Note
You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more
information about EtherChannel, see
Layer 2 Interface Modes
Table 18-1
Table 18-1
Mode
switchport mode access
switchport mode
dynamic desirable
switchport mode dynamic auto
switchport mode trunk
switchport nonegotiate
DTP is a point-to-point protocol. However, some internetworking devices might forward DTP frames
Note
improperly. To avoid this problem, 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 without generating DTP frames.
OL_28731-01
Ports 3 to 18 are blocking Gigabit ports on the WS-X4418-GB module. Ports 1 to 12 are blocking
Gigabit ports on the WS-X4412-2GB-T module.
lists the Layer 2 interface modes and describes how they function on Ethernet interfaces.
Layer 2 Interface Modes
Chapter 26, "Configuring EtherChannel and Link State Tracking."
Purpose
Puts the interface into permanent nontrunking mode and
negotiates to convert the link into a nontrunking link. The
interface becomes a nontrunk interface even if the neighboring
interface does not change.
Makes the interface actively attempt to convert the link to a
trunking link. The interface becomes a trunk interface if the
neighboring interface is set to trunk, desirable, or auto mode.
Makes the interface convert the link to a trunking link if the
neighboring interface is set to trunk or desirable mode. It is the
default mode for all Ethernet interfaces.
Puts the interface into permanent trunking mode and negotiates to
convert the link into a trunking link. The interface becomes a
trunk interface even if the neighboring interface does not change.
Puts the interface into permanent trunking mode but prevents the
interface from generating DTP frames. You must configure the
neighboring interface manually as a trunk interface to establish a
trunking link.
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)E
About Layer 2 Ethernet Switching
18-3