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Catalyst 3750 MetroSwitch Software Configuration Guide
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Chapter 10 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.
• On an ES port, you can use the switchport trunk dot1q ethertype value interface configuration
command to program a custom ethertype value for the interface. This feature allows the switch to
interoperate with third-party vendor switches that do not use the standard 0x8100 ethertype to
identify 802.1Q-tagged frames. For example, if you set 0x1234 as the custom 802.1Q ethertype on
a trunk port, incoming frames containing that ethertype are assigned to the VLAN contained in the
tag following the ethertype as they would be with a standard 802.1Q trunk. Frames arriving on a
trunk port with either the standard ethertype (0x8100) or the custom ethertype value are treated as
valid 802.1Q traffic; frames containing any other ethertype are assigned to the native VLAN of that
trunk.
Default Layer 2 Ethernet Interface VLAN Configuration
Table 10-6 shows the default Layer 2 Ethernet interface VLAN configuration.
Table 10-6 Default Layer 2 Ethernet Interface VLAN Configuration
Feature Default Setting
Interface mode switchport mode dynamic auto
Trunk encapsulation switchport trunk encapsulation negotiate
802.1Q encapsulation ethertype value 0x8100
Allowed VLAN range VLANs 1 to 4094
VLAN range eligible for pruning VLANs 2 to 1001
Default VLAN (for access ports) VLAN 1
Native VLAN (for 802.1Q trunks) VLAN 1
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