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Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
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Chapter12 Configuring VLANs
Configuring VLAN Trunks
To return an interface to its default configuration, use the default interface interface-id interface
configuration command. To reset all trunking characteristics of a trunking interface to the defaults, us e
the no switchport trunk interface configuration command. To disable trunking, use the switchport
mode access interface configuration command to configure the port as a static-access port.
This example shows how to configure a port as an IEEE 802.1Q trunk. The exa mple assumes that the
neighbor interface is configured to support IEEE 802.1Q trunking.
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface gigabitethernet0/17
Switch(config-if)# switchport mode dynamic desirable
Switch(config-if)# end
Defining the Allowed VLANs on a Trunk
Note You cannot change the trunk mode on the internal interfaces connected to t he 100 Mbps management
module (ports 15 and 16). You also cannot remove the management VLAN from the allowed list.
By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs 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.
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. This is known as VLAN 1 minimization. VLAN 1
minimization disables VLAN 1 (the default VLAN on all Cisco switch trunk ports) on an individual
VLAN trunk link. As a result, no user traffic, including spanning-tree advertisements, is sent or received
on VLAN1.
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), Link
Aggregation Control Protocol (LACP), Dynamic Trunking Protocol (DTP), and VLAN Trunking
Protocol (VTP) 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 VLAN1, 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, if VTP knows of the VLAN,
and if the VLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the
VLAN is in the allowed list for a trunk port, the trunk port automatically becomes a member of the
enabled VLAN. When VTP detects a new VLAN and the VLAN is not in the allowed list for a trunk
port, the trunk port does not become a member of the new VLAN.
Step8 show interfaces interface-id trunk Display the trunk configuration of the interface.
Step9 copy running-config startup-config (Optional) Save your entries in the configuration file.
Command Purpose