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Catalyst 2900 Series XL and Catalyst 3500 SeriesXL Software Configuration Guide
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Chapter8 Configuring VLANs
How VLAN Trunks Work
How VLAN Trunks Work
A trunk is a point-to-point link that sends and receives traffic between switches or between switches and
routers. Trunks carry the traffic of multiple VLANs and can extend VLANs across an entire network.
100BASE-T and Gigabit Ethernet trunks use Cisco Inter-Switch Link (ISL), the default protocol, or
industry-standard IEEE 802.1Q to carry traffic for multiple VLANs over a single link.
Figure 8-4 shows a network of switches that are connected by ISL trunks.
Figure 8-4 Catalyst 2900 XL and Catalyst 3500 XL Switches in an ISL Trunking Environment
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 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.
Catalyst 5000 series
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