Verifying Trunks - Cisco Catalyst 2000 Configuration Handbook

By specifying VLANs in the vlanlist field of this command, the VLANs will not be
allowed to travel across the trunk link until they are added back to the trunk using
the command set trunk mod/port vlanlist or switchport trunk allowed vlan add
vlanlist.

Verifying Trunks

After configuring a port for trunking, use the following command to verify the VLAN
port assignments:
Router# show interfaces trunk
Feature Example
In this example the switches Access_1, Distribution_1, and Core_1 are connected as
shown in Figure 6-1. 802.1Q trunking is configured in the on mode between Access_1
and Distribution_1 switches. ISL is configured in desirable mode on the Distribution_1
switch to the link connecting to the core. The core is configured for autotrunking mode
and encapsulation negotiate. The trunk connected between the access switch is config-
ured to only trunk for VLANs 5, 8, and 10. The trunk between the Distribution_1 and
Core_1 is configured to carry only VLAN 1 and VLAN 10.
Figure 6-1 Network Diagram for Trunk
Configuration on Access_1, Distribution_1,
and Core_1
An example of the Catalyst IOS configuration for Distribution_1 follows:
Distribution_1 (config#) interface gigabitethernet 2/1
Distribution_1 (config-if)# switchport mode trunk
Distribution_1 (config-if)# switchport trunk encapsulation dot1q
Distribution_1 (config-if)# switchport trunk allowed vlan allowed 5,8,10
Distribution_1 (config-if)# end
Distribution_1# copy running-config startup-config
An example of IOS configuration for Core_1 follows:
Core_1(config)# interface gigabitethernet 1/1
Distribution_1
Access_1
802.1Q Trunk
G 0/1 2/1
Chapter 6: VLANs and Trunking 97
Si
1/1
Core_1
ISL Trunk
1/1
Si