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Chapter 5
Configuring Ethernet VLAN Trunks
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Default Trunk Configuration
Table 5-4
Table 5-4
Feature
Trunk mode
Trunk encapsulation
Allowed VLAN range
1. With software release 8.3(1) and later releases, instead of reserved VLANs, we now have only user and internal VLANs. VLAN
Configuring a Trunk Link
These sections describe how to configure a trunk link on Ethernet ports and how to define the allowed
VLAN range on a trunk:
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OL-8978-04
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).
When you connect a Cisco switch to a non-Cisco switch, the CST is always on VLAN 1. The Cisco
switch sends an untagged IEEE BDPU (01-80-C2-00-00-00) on VLAN 1 for the CST. On the native
VLAN, the Cisco switch sends an untagged Cisco BPDU (01-00-0C-CC-CC-CC) which the
non-Cisco switch forwards but does not act on (the IEEE BPDU is not forwarded on the native
VLAN).
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 that are connected to the
non-Cisco 802.1Q cloud receive these flooded BPDUs. This situation 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 that are connected to the non-Cisco 802.1Q cloud through the 802.1Q trunks.
Make sure that the native VLAN is the same on all of the 802.1Q trunks connecting the Cisco
switches to the non-Cisco 802.1Q cloud.
If you are connecting multiple Cisco switches to a non-Cisco 802.1Q cloud, all of the connections
must be through 802.1Q trunks. You cannot connect Cisco switches to a non-Cisco 802.1Q cloud
through ISL trunks or through access ports because the switch will place the ISL trunk port or access
port into the spanning tree "port inconsistent" state and no traffic will pass through the port.
shows the default Ethernet trunk configuration.
Default Ethernet Trunk Configuration
manager no longer permanently sets aside VLANs for features that require them; they are now dynamically assigned as needed.
The entire VLAN range (1 to 4094) is now available for user (and internal) VLANs.
Configuring an ISL Trunk, page 5-6
Configuring an 802.1Q Trunk, page 5-7
Configuring an ISL/802.1Q Negotiating Trunk Port, page 5-8
Default Configuration
auto
negotiate
VLANs 1–1005, 1025–4094
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
Default Trunk Configuration
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