Spanning Tree And Switch Stacks; Default Spanning-Tree Configuration - Cisco Catalyst 2960-XR Configuration Manual

Configuring Spanning Tree Protocol
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
• When a new switch joins the stack, it sets its bridge ID to the stack master bridge ID. If the newly added
• When a stack member leaves the stack, spanning-tree reconvergence occurs within the stack (and possibly
• If the stack master fails or leaves the stack, the stack members elect a new stack master, and all stack
• If the switch stack is the spanning-tree root and the stack master fails or leaves the stack, the stack
• If a neighboring switch external to the switch stack fails or is powered down, normal spanning-tree
• If a new switch external to the switch stack is added to the network, normal spanning-tree processing

Default Spanning-Tree Configuration

Table 6: Default Spanning-Tree Configuration
Feature
Enable state
Spanning-tree mode
Switch priority
Spanning-tree port priority (configurable on a per-interface basis)
OL-29424-01
use the same bridge ID for a given spanning tree. The bridge ID is derived from the MAC address of
the stack master.
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.
outside the stack). The remaining stack member with the lowest stack port ID becomes the stack root.
members change their bridge IDs of the spanning trees to the new master bridge ID.
members elect a new stack master, and a spanning-tree reconvergence occurs.
processing occurs. Spanning-tree reconvergence might occur as a result of losing a switch in the active
topology.
occurs. Spanning-tree reconvergence might occur as a result of adding a switch in the network.
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
Default Spanning-Tree Configuration
Default Setting
Enabled on VLAN 1.
PVST+. (Rapid PVST+ and MSTP
are disabled.)
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