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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 8 Configuring VLANs
How VLAN Trunks Work
IEEE 802.1Q Configuration Considerations
IEEE 802.1Q trunks impose these limitations on the trunking strategy for a network:
• Make sure the native VLAN for a 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.
• Make sure your network is loop-free before disabling STP.
Note The Catalyst 2950 switches do not support ISL trunking.
Trunks Interacting with Other Features
IEEE 802.1Q trunking interacts with other switch features as described in Table 8-8.
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 Puts the interface into permanent trunking mode but prevents the interface from
generating DTP frames. You must manually configure the neighboring interface as a
trunk interface to establish a trunk link.
Table 8-7 Interface Modes (continued)
Mode Function
Table 8-8 Trunks Interacting with Other Features
Switch Feature Trunk Port Interaction
Secure ports A trunk port cannot be a secure port.
Port grouping 802.1Q trunks can be grouped into EtherChannel port groups, but all trunks in the
group must have the same configuration.
When a group is first created, all ports follow the parameters set for the first port
to be added to the group. If you change the configuration of one of these
parameters, the switch propagates the setting you entered to all ports in the group:
• Allowed-VLAN list
• STP path cost for each VLAN
• STP port priority for each VLAN
• STPPortFastsetting
• Trunk status: if one port in a port group ceases to be a trunk, all port cease to
be trunks.
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