Spanning Tree Protocols and IEEE 802.1Q Trunks The IEEE 802.1Q standard for VLAN trunks imposes some limitations on the spanning-tree strategy for a network. The standard requires only one spanning-tree instance for all VLANs allowed on the trunks. However, in a network of Cisco devices that are connected through IEEE 802.1Q trunks, the devices maintain one spanning-tree instance for each VLAN allowed on the trunks. When you connect a Cisco device to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco device uses PVST+ to provide spanning-tree interoperability. If Rapid PVST+ is enabled, the device uses it instead of PVST+.
The device combines the spanning-tree instance of the IEEE 802.1Q VLAN of the trunk with the spanning-tree instance of the non-Cisco IEEE 802.1Q device. However, all PVST+ or Rapid PVST+ information is maintained by Cisco devices that are separated by a cloud of non-Cisco IEEE 802.1Q devices. The non-Cisco IEEE 802.1Q cloud separating the Cisco devices is treated as a single trunk link between the devices. Rapid PVST+ is automatically enabled on IEEE 802.1Q trunks, and no user configuration is required. The external spanning-tree behavior on access ports and Inter-Switch Link (ISL) trunk ports is not affected by PVST+.
Spanning Tree and Switch Stacks When the switch stack is operating in PVST+ or Rapid PVST+ mode: • A switch stack appears as a single spanning-tree node to the rest of the network, and all stack members use the same bridge ID for a given spanning tree. The bridge ID is derived from the MAC address of the active switch. • When a new device joins the stack, it sets its bridge ID to the active switch bridge ID. If the newly added device has the lowest ID and if the root path cost is the same among all stack members, the newly added device becomes the stack root. • When a stack member leaves the stack, spanning-tree reconvergence occurs within the stack (and possibly outside the stack).
The remaining stack member with the lowest stack port ID becomes the stack root. • If the switch stack is the spanning-tree root and the active switch fails or leaves the stack, the standby switch becomes the new active switch, bridge IDs remain the same, and a spanning-tree reconvergence might occur. • If a neighboring device external to the switch stack fails or is powered down, normal spanning-tree processing occurs. Spanning-tree reconvergence might occur as a result of losing a device in the active topology. • If a new device external to the switch stack is added to the network, normal spanning-tree processing occurs.
Spanning-tree reconvergence might occur as a result of adding a device in the network. Default Spanning-Tree Configuration Table 3: Default Spanning-Tree Configuration Default Setting Feature Enabled on VLAN 1. Enable state Configuring Spanning Tree Protocol Spanning Tree Protocols and IEEE 802.1Q Trunks Configuring Spanning Tree Protocol 10