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Catalyst 2960 Switch SoftwareConfiguration Guide
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Chapter12 Configuring VLANs
Configuring VLAN Trunks
IEEE 802.1Q Configuration Considerations
The IEEE 802.1Q trunks impose these limitations on the trun king strategy for a network:
•In a network of Cisco switches connected through IEEE 802.1Q trun ks, the switches maintain one
spanning-tree instance for each VLAN allowed on the trunks. Non-Cisco devices might suppo rt one
spanning-tree instance for all VLANs.
When you connect a Cisco switch to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco
switch combines the spanning-tree instance o f the VLAN of the trunk with the spanning-tree
instance of the non-Cisco IEEE 802.1Q switch. However, spanning-tree information for each VLAN
is maintained by Cisco switches separated by a cloud of non-Cisco IEE E 802.1Q switches. The
non-Cisco IEEE 802.1Q cloud separating the Cisco switches is tr eated as a single trunk link between
the switches.
•Make sure the native VLAN for an IEEE 802.1Q trunk is the same on both ends of the trunk link. If
the native VLAN on one end of the trunk is different from the native VLAN on the other end,
spanning-tree loops might result.
•Disabling spanning tree on the native VLAN of an IEEE 802.1Q trunk without disabling spanning
tree on every VLAN in the network can potentially cause spanning-tree loops. We recommend that
you leave spanning tree enabled on the native VLAN of an IEEE 802.1Q trunk or d isable spanning
tree on every VLAN in the network. Make sure your network is loop-free before disabling spannin g
tree.
Table12-4 Layer 2 Interface Modes
Mode Function
switchport mode access Puts the interface (access port) into permanent nontrunking mode and negotiates to
convert the link into a nontrunk link. The interface becomes a nontrunk interface
regardless of whether or not the neighboring interface is a trunk interface.
switchport mode dynamic auto Makes the interface able to convert the link to a trunk link. The interface becomes a trunk
interface if the neighboring interface is set to trunk or desirable mode. The default
switchport mode for all Ethernet interfaces is dynamic auto.
switchport mode dynamic
desirable
Makes the interface actively attempt to convert the link to a trunk link. The interface
becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto
mode.
switchport mode trunk Puts the interface into permanent trunking mode and negotiates to convert the
neighboring link into a trunk link. The interface becomes a trunk interface even if the
neighboring interface is not a trunk interface.
switchport nonegotiate Prevents the interface from generating DTP frames. You can use this command only
when the interface switchport mode is access or trunk. You must manually configure the
neighboring interface as a trunk interface to establish a trunk link.