11-6
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 11 Configuring Layer 2 Ethernet Interfaces
Configuring Ethernet Interfaces for Layer 2 Switching
Configuring an Ethernet Interface as a Layer 2 Trunk
Note The default for Layer 2 interfaces is switchport mode dynamic auto. If the neighboring interface
supports trunking and is configured to trunk mode or dynamic desirable mode, the link becomes a
Layer 2 trunk. By default, trunks negotiate encapsulation. If the neighboring interface supports ISL and
802.1Q encapsulation and both interfaces are set to negotiate the encapsulation type, the trunk uses ISL
encapsulation.
To configure an interface as a Layer 2 trunk, perform this task:
Command Purpose
Step 1
Switch(config)# interface {fastethernet |
gigabitethernet | tengigabitethernet}
slot
/
port
Specifies the interface to configure.
Step 2
Switch(config-if)# shutdown
(Optional) Shuts down the interface to prevent traffic flow until
configuration is complete.
Step 3
Switch(config-if)# switchport trunk
encapsulation {isl | dot1q | negotiate}
(Optional) Specifies the encapsulation.
Note You must enter this command with either the isl or dot1q
keyword to support the switchport mode trunk
command, which is not supported by the default mode
(negotiate).
Step 4
Switch(config-if)# switchport mode
{dynamic {auto | desirable} | trunk}
Configures the interface as a Layer 2 trunk. (Required only if the
interface is a Layer 2 access port or to specify the trunking mode.)
Step 5
Switch(config-if)# switchport access vlan
vlan_num
(Optional) Specifies the access VLAN, which is used if the
interface stops trunking. The access VLAN is not used as the
native VLAN.
Note The vlan_num parameter is either a single VLAN number
from 1 to 1005 or a range of VLANs described by two
VLAN numbers, the lesser one first, separated by a dash.
Do not enter any spaces between comma-separated vlan
parameters or in dash-specified ranges.
Step 6
Switch(config-if)# switchport trunk
native vlan
vlan_num
For 802.1Q trunks, specifies the native VLAN.
Note If you do not set the native VLAN, the default is used
(VLAN 1).
Step 7
Switch(config-if)# switchport trunk
allowed vlan {add | except | all |
remove}
vlan_num
[,
vlan_num
[,
vlan_num
[,....]]
(Optional) Configures the list of VLANs allowed on the trunk. All
VLANs are allowed by default. You cannot remove any of the
default VLANs from a trunk.
Step 8
Switch(config-if)# switchport trunk
pruning vlan {add | except | none |
remove}
vlan_num
[,
vlan_num
[,
vlan_num
[,....]]
(Optional) Configures the list of VLANs allowed to be pruned
from the trunk (see the “Understanding VTP Pruning” section on
page 27-3). The default list of VLANs allowed to be pruned
contains all VLANs, except for VLAN 1.
Step 9
Switch(config-if)# no shutdown
Activates the interface. (Required only if you shut down the
interface.)
Step 10
Switch(config-if)# end
Exits interface configuration mode.
