Related Topics
Configuring the Root Switch, on page 29
Bridge ID, Device Priority, and Extended System ID, on page 17
Spanning-Tree Topology and BPDUs, on page 15
Accelerated Aging to Retain Connectivity, on page 22
Information About Spanning Tree Protocol
Spanning Tree Protocol
Spanning Tree Protocol (STP) is a Layer 2 link management protocol that provides path redundancy while
preventing loops in the network. For a Layer 2 Ethernet network to function properly, only one active path
can exist between any two stations. Multiple active paths among end stations cause loops in the network. If
a loop exists in the network, end stations might receive duplicate messages. Switches might also learn
end-station MAC addresses on multiple Layer 2 interfaces. These conditions result in an unstable network.
Spanning-tree operation is transparent to end stations, which cannot detect whether they are connected to a
single LAN segment or a switched LAN of multiple segments.
The STP uses a spanning-tree algorithm to select one switch of a redundantly connected network as the root
of the spanning tree. The algorithm calculates the best loop-free path through a switched Layer 2 network by
assigning a role to each port based on the role of the port in the active topology:
• Root—A forwarding port elected for the spanning-tree topology
• Designated—A forwarding port elected for every switched LAN segment
• Alternate—A blocked port providing an alternate path to the root bridge in the spanning tree
• Backup—A blocked port in a loopback configuration
The switch that has all of its ports as the designated role or as the backup role is the root switch. The switch
that has at least one of its ports in the designated role is called the designated switch.
Spanning tree forces redundant data paths into a standby (blocked) state. If a network segment in the spanning
tree fails and a redundant path exists, the spanning-tree algorithm recalculates the spanning-tree topology and
activates the standby path. Switches send and receive spanning-tree frames, called bridge protocol data units
(BPDUs), at regular intervals. The switches do not forward these frames but use them to construct a loop-free
path. BPDUs contain information about the sending switch and its ports, including switch and MAC addresses,
switch priority, port priority, and path cost. Spanning tree uses this information to elect the root switch and
root port for the switched network and the root port and designated port for each switched segment.
When two ports on a switch are part of a loop, the spanning-tree and path cost settings control which port is
put in the forwarding state and which is put in the blocking state. The spanning-tree port priority value
represents the location of a port in the network topology and how well it is located to pass traffic. The path
cost value represents the media speed.
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
14 OL-29424-01
Configuring Spanning Tree Protocol
Information About Spanning Tree Protocol
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