Cisco Catalyst 2960 Series, Configuration Manual

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VLAN-bridge spanning tree allows the bridge groups to form a spanning tree on top of the 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.
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 switchstack master.
• When a new switch joins the stack, it sets its bridge ID to the active switchstack 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 the switch stack is the spanning-tree root and the active switchstack master 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 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.
Default Spanning-Tree Configuration
Table 27: Default Spanning-Tree Configuration
Default Setting Feature
Enabled on VLAN 1. Enable state
Rapid PVST+ ( PVST+ and MSTP
are disabled.) Spanning-tree mode
32768 Switch priority
128 Spanning-tree port priority (configurable on a per-interface basis)
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
223
Information About Spanning Tree Protocol