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Understanding How VLAN Trunks Work
Trunking Support
Trunking capabilities are hardware-dependent.
hardware that supports the two trunking encapsulations. To determine whether a specific piece of
hardware supports trunking, and to determine which trunking encapsulations are supported, see your
hardware documentation or use the show port capabilities command.
Table 11-4 Trunking Encapsulation Support
Trunking Method
ISL
802.1Q
Negotiate
802.1Q Trunk Restrictions
The following configuration guidelines and restrictions apply when using 802.1Q trunks impose some
limitations on the trunking strategy for a network. Note these restrictions when using 802.1Q trunks:
•
•
•
•
•
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
11-4
When connecting Cisco switches through an 802.1Q trunk, 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 spanning tree on the native VLAN of an 802.1Q trunk without disabling spanning tree on
every VLAN in the network can cause spanning-tree loops. We recommend that you leave spanning
tree enabled on the native VLAN of an 802.1Q trunk. If this is not possible, disable spanning tree
on every VLAN in the network. Make sure your network is free of physical loops before disabling
spanning tree.
When you connect two Cisco switches through 802.1Q trunks, the switches exchange spanning-tree
BPDUs on each VLAN allowed on the trunks. The BPDUs on the native VLAN of the trunk are sent
untagged to the reserved IEEE 802.1d spanning-tree multicast MAC address (01-80-C2-00-00-00).
The BPDUs on all other VLANs on the trunk are sent tagged to the reserved Cisco Shared Spanning
Tree (SSTP) multicast MAC address (01-00-0c-cc-cc-cd).
Non-Cisco 802.1Q switches maintain only a single instance of spanning tree (the Mono Spanning
Tree, or MST) that defines the spanning-tree topology for all VLANs. When you connect a Cisco
switch to a non-Cisco switch through an 802.1Q trunk, the MST of the non-Cisco switch and the
native VLAN spanning-tree of the Cisco switch combine to form a single spanning-tree topology
known as the Common Spanning Tree (CST).
Because Cisco switches transmit BPDUs to the SSTP multicast MAC address on VLANs other than
the native VLAN of the trunk, non-Cisco switches do not recognize these frames as BPDUs and
flood them on all ports in the corresponding VLAN. Other Cisco switches connected to the
non-Cisco 802.1Q cloud receive these flooded BPDUs. This allows Cisco switches to maintain a
per-VLAN spanning-tree topology across a cloud of non-Cisco 802.1Q switches. The non-Cisco
802.1Q cloud separating the Cisco switches is treated as a single broadcast segment between all
switches connected to the non-Cisco 802.1Q cloud through 802.1Q trunks.
Chapter 11
Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports
Table 11-4
Catalyst 4000
Catalyst 2948G
Family
Catalyst 2980G
No
No
Yes
Yes
No
No
shows which switches have available
78-12647-02
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Troubleshooting
