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13-13
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter 13 Configuring VLANs
Configuring VLAN Trunks
802.1Q Configuration Considerations
802.1Q trunks impose these limitations on the trunking strategy for a network:
• In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance
of spanning tree for each VLAN allowed on the trunks. Non-Cisco devices might support one
spanning-tree instance for all VLANs.
When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch
combines the spanning-tree instance of the VLAN of the trunk with the spanning-tree instance of
the non-Cisco 802.1Q switch. However, spanning-tree information for each VLAN is maintained by
Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud
separating the Cisco switches is treated as a single trunk link between the switches.
• Make sure the native VLAN for an 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 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 802.1Q trunk or disable spanning tree on
every VLAN in the network. Make sure your network is loop-free before disabling spanning tree.
Default Layer 2 Ethernet Interface VLAN Configuration
Table 13-5 shows the default Layer 2 Ethernet interface VLAN configuration.
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.
switchport mode trunk Puts the interface into permanent trunking mode and negotiates to convert the 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.
Table 13-4 Layer 2 Interface Modes (continued)
Mode Function
Table 13-5 Default Layer 2 Ethernet Interface VLAN Configuration
Feature Default Setting
Interface mode switchport mode dynamic desirable
Allowed VLAN range VLANs 1 to 1005
VLAN range eligible for pruning VLANs 2 to 1001
Default VLAN (for access ports) VLAN 1
Native VLAN (for 802.1Q trunks) VLAN 1