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Chapter17 Configuring STP
Configuring Spanning-Tree Features
VLAN spanning trees to prevent loops from forming if there are mult iple co nne ctio ns am ong VLA Ns .
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 a re in cre as ed. To use the
fallback bridging feature, you must have the IP services feature set en ab led on yo ur sw itch. For m ore
information, see Chapter 46, “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 bridg e 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 s tack mem bers ele ct a ne w s tack ma ster , and all s tack
members change their bridge IDs of the spanning trees to the n ew master br idge I D.
•If the switch stack is the spanning-tree root and the stack master fail s or leaves the stac k, the st ac k
members elect a new stack master, and a spanning-tree re convergence occ ur s.
•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, norma l 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 Chapter5, “Managing Switch Stacks.”
Configuring Spanning-Tree Features
These sections contain this configuration information:
•Default Spanning-Tree Configuration, page 17-13
•Spanning-Tree Configuration Guidelines, page 17-13
•Changing the Spanning-Tree Mode., page 17-15 (required)
•Disabling Spanning Tree, page 17-16 (optional)
•Configuring the Root Switch, page 17-16 (optional)
•Configuring a Secondary Root Switch, page 17-18 (optional)
•Configuring Port Priority, page 17-18 (optional)
•Configuring Path Cost, page 17-20 (optional)