Configuring Spanning-Tree Features, 18-12

Chapter 18 Configuring STP

Configuring Spanning-Tree Features

individual VLAN spanning trees to prevent loops from forming if there are multiple connections among VLANs. It also prevents the individual spanning trees from the VLANs being bridged from collapsing into a single spanning tree.

To support VLAN-bridge spanning tree, some of the spanning-tree timers are increased. To use the fallback bridging feature, you must have the IP services feature set enabled on your switch. For more information, see Chapter 44, “Configuring Fallback Bridging.”

Spanning Tree and Switch Stacks

These statements are true 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 stack master.

•When a new switch joins the stack, it sets its bridge ID to the stack-master bridge ID. If the newly added switch has the lowest ID and if the root path cost is the same among all stack members, the newly added switch 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 stack master fails or leaves the stack, the stack members elect a new stack master, and all stack members change their bridge IDs of the spanning trees to the new master bridge ID.

•If the switch stack is the spanning-tree root and the stack master fails or leaves the stack, the stack members elect a new stack master, and a spanning-tree reconvergence occurs.

•If a neighboring switch 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 switch in the active topology.

•If a new switch 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 switch in the network.

For more information about switch stacks, see Chapter 5, “Managing Switch Stacks.”