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Cisco Systems Intelligent Gigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide
24R9746
Chapter 12 Configuring VLANs
Configuring VLAN Trunks
IEEE 802.1Q Configuration Considerations
IEEE 802.1Q trunks impose these limitations on a network:
• In a network of Cisco switches connected through IEEE 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 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 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 your network is loop-free before disabling spanning
tree.
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 12-4 Layer 2 Interface Modes (continued)
Mode Function