Cisco ME 3400G-2CS - Ethernet Access Switch Software Configuration Manual page 239

Configuring VLAN Trunks
Note VLAN 1 is the default VLAN on all trunk ports in all Cisco switches, and it has previously been a
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
no shutdown
Step 4
switchport mode trunk
Step 5 switchport trunk allowed vlan {add |
all | except | remove} vlan-list
Step 6 end
Step 7
show interfaces interface-id switchport Verify your entries in the Trunking VLANs Enabled field of the display.
Step 8
copy running-config startup-config
To return to the default allowed VLAN list of all VLANs, use the no switchport trunk allowed vlan
interface configuration command.
This example shows how to remove VLAN 2 from the allowed VLAN list on a port:
Switch(config)# interface fastethernet0/1
Switch(config-if)# switchport trunk allowed vlan remove 2
Switch(config-if)# end
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Purpose
Enter global configuration mode.
Specify the port to be configured, and enter interface configuration
mode.
Enable the port, if necessary. By default, UNIs are disabled, and NNIs are
enabled.
Configure the interface as a VLAN trunk port.
(Optional) Configure the list of VLANs allowed on the trunk.
For explanations about using the add, all, except, and remove keywords,
see the command reference for this release.
The vlan-list parameter is either a single VLAN number from 1 to 4094
or a range of VLANs described by two VLAN numbers, the lower one
first, separated by a hyphen. Do not enter any spaces between
comma-separated VLAN parameters or in hyphen-specified ranges.
All VLANs are allowed by default.
Return to privileged EXEC mode.
(Optional) Save your entries in the configuration file.
Chapter 11 Configuring VLANs
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