802.1Q Trunk Configuration Guidelines and Restrictions
The following configuration guidelines and restrictions apply when using 802.1Q trunks and impose some limitations on the trunking strategy for a network:
•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 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 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 that is 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).
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.
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_8_5/confg_gd/e_trunk.htm#wp1045882
