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Catalyst 2900 Series XL and Catalyst 3500 Series XL 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 some limitations on the trunking strategy for a
network. The following restrictions apply when using 802.1Q trunks:
• 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 STP 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.
Trunks Interacting with Other Features
ISL, IEEE 802.1Q, and ATM trunking interacts with other switch features as
described in Table 8-11.
Table 8-11 Trunks Interacting with Other Features
Switch Feature Trunk Port Interaction
Port monitoring A trunk port cannot be a monitor port. A static-access port
can monitor the traffic of its VLAN on a trunk port.
Network port When configured as a network port, a trunk port serves as
the network port for all VLANs associated with the port. A
network port receives all unknown unicast traffic on a
VLAN.
Secure ports A trunk port cannot be a secure port.
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