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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 9 Creating and Maintaining VLANs
Understanding 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 STP on the native VLAN of an 802.1Q trunk without disabling STP on every VLAN in
the network can potentially cause spanning-tree loops. We recommend that you leave STP enabled
on the native VLAN of an 802.1Q trunk or disable STP on every VLAN in the network. Make sure
your network is loop-free before disabling STP.
Default Layer 2 Ethernet Interface VLAN Configuration
Table 9-8
shows the default Layer 2 Ethernet interface VLAN configuration.
Table 9-8
Default Layer 2 Ethernet Interface VLAN Configuration
Feature
Default Setting
Interface mode
switchport mode dynamic desirable
Trunk encapsulation
switchport trunk encapsulation negotiate
Allowed VLAN range
VLANs 1–1005
VLAN range eligible for pruning
VLANs 2–1001
Default VLAN (for access ports)
VLAN 1
Native VLAN (for 802.1Q trunks)
VLAN 1
