10-10
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 10 Configuring VLANs
Configuring VLAN Trunks
IEEE 802.1Q Configuration Considerations
The IEEE 802.1Q trunks impose these limitations on the trunking strategy for a network:
• In a network of Cisco switches connected through IEEE 802.1Q trunks, the switches maintain one
spanning-tree instance 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 IEEE 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 IEEE 802.1Q switch. However, spanning-tree information for each VLAN
is maintained by Cisco switches separated by a cloud of non-Cisco IEEE 802.1Q switches. The
non-Cisco IEEE 802.1Q cloud separating the Cisco switches is treated as a single trunk link between
the switches.
• Make sure that 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 disable spanning
tree on every VLAN in the network. Make sure that your network is loop-free before disabling
spanning tree.
Default Layer 2 Ethernet Interface VLAN Configuration
Table 10-5 shows the default Layer 2 Ethernet interface VLAN configuration.
Table 10-4 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. This is the
default 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.
Table 10-5 Default Layer 2 Ethernet Interface VLAN Configuration
Feature Default Setting
Interface mode switchport mode access
Allowed VLAN range VLANs 1 to 4094
Default VLAN (for access ports) VLAN 1
Native VLAN (for IEEE 802.1Q trunks) VLAN 1
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