802.1Q Trunk Restrictions - Cisco WS-C2950SX-48-SI Configuration Manual

Understanding How VLAN Trunks Work
Table 11-4 Trunking Encapsulation Support
Trunking Method
ISL
802.1Q
Negotiate

802.1Q Trunk Restrictions

This section lists the configuration guidelines and restrictions for using 802.1Q trunks to impose some
limitations on the trunking strategy for a network. These restrictions apply when using 802.1Q trunks:
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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
11-4
For a trunk to come up and work, you must physically connect the trunk port to another network
device.
When using VTP to carry VLANs over the trunk port, you must manually configure extended
VLANs on each switch, because VTP carries only VLANs 1 1005.
When connecting Cisco switches through an 802.1Q trunk, make sure that 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 can 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 that 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.
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.
Chapter 11 Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports
Catalyst 4000 Catalyst 2948G
Series Catalyst 2980G
No No
Yes Yes
No No
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Release 8.1
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