Defining The Allowed Vlans On A Trunk - Cisco 3845 - Security Bundle Router Software Manual

Configuring VLAN Trunks
This example shows how to configure a port as an IEEE 802.1Q trunk with VLAN 33 as the native
VLAN:
Switch# configure terminal
Enter configuration commands, one per line.
Switch(config)# interface gigabitethernet/2
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk native vlan 33
Switch(config-if)# end

Defining the Allowed VLANs on a Trunk

By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs, 1 to 4094,
are allowed on each trunk. However, you can remove VLANs from the allowed list, preventing traffic
from those VLANs from passing over the trunk. To restrict the traffic a trunk carries, use the switchport
trunk allowed vlan remove vlan-list interface configuration command to remove specific VLANs from
the allowed list.
VLAN 1 is the default VLAN on all trunk ports in all Cisco switches, and it has previously been a
Note
requirement that VLAN 1 always be enabled on every trunk link. The VLAN 1 minimization feature
allows you to disable VLAN 1 on any individual VLAN trunk link so that no user traffic (including
spanning-tree advertisements) is sent or received on VLAN 1. You do this by removing VLAN 1 from
the allowed VLAN list.
To reduce the risk of spanning-tree loops or storms, you can disable VLAN 1 on any individual VLAN
trunk port by removing VLAN 1 from the allowed list. When you remove VLAN 1 from a trunk port,
the interface continues to send and receive management traffic, for example, Cisco Discovery Protocol
(CDP), Port Aggregation Protocol (PAgP), and Link Aggregation Control Protocol (LACP) in VLAN 1.
If a trunk port with VLAN 1 disabled is converted to a nontrunk port, it is added to the access VLAN. If
the access VLAN is set to 1, the port is added to VLAN 1, regardless of the switchport trunk allowed
setting. The same is true for any VLAN that has been disabled on the port.
A trunk port can become a member of a VLAN if the VLAN is enabled and if the VLAN is in the allowed
list for the port.
Beginning in privileged EXEC mode, follow these steps to modify the allowed list of an
IEEE 802.1Q trunk:
Command
Step 1 configure terminal
Step 2 interface interface-id
Step 3
switchport mode trunk
Cisco ME 3800X and 3600X Switch Software Configuration Guide
10-12
End with CNTL/Z.
Purpose
Enter global configuration mode.
Specify the port to be configured, and enter interface configuration
mode.
Configure the interface as a VLAN trunk port.
Chapter 10 Configuring VLANs
OL-23400-01