28.1.4 Components of a spanning-tree topology

Designing a Spanning Tree topology involves roles for bridges and ports, as well as parameters used to determine these roles. The following explains usage for these configuration components and topology designs.

<Structure of this section>

(1) Bridge Role

The table below describes bridge roles. Spanning Tree topology design starts with determining the root bridge.

Table 28-5: Roles of bridges
Bridge role	Overview
Root bridge	The switch at the logical center of a built topology. There can only be one within a topology.
Designated bridge	A switch other than the root bridge for forwarding frames from the root bridge.

(2) Role of the port

The table below describes port roles. Ports on designated bridges have three types of roles. For root bridges, all ports are designated ports.

Table 28-6: Roles of ports
Port role	Overview
Root port	A port for a communication path from a designated bridge to the root bridge. This port allows communication.
Designated port	A port, other than the root port, for which communication is possible. It allows communication downstream from the root bridge to other ports in the topology.
Non-designated port	A port other than a root port or designated port, for which communication is not possible. It serves as an alternate path when a fault occurs.

(3) Bridge identifier

Each switch in a topology is identified by a parameter called a bridge ID. The switch that has the lowest bridge ID has the highest priority, and is selected as the root bridge.

Bridge IDs consist of a bridge priority (16 bits) and the bridge MAC address (48 bits). The lowest 12 bits of a bridge priority is the extended system ID. For an extended system ID, 0 is set for Single Spanning Tree or Multiple Spanning Tree, and the VLAN ID is set for PVST+. The following figure shows a bridge ID.

Figure 28-2: Bridge identifier

(4) Path cost

A value corresponding to the communication speed of each port on a switch is called the path cost. The total value of the port costs for all intermediate ports from a designated bridge to the root bridge is called the root path cost. If there are multiple paths to the root bridge, the root path cost is that of the shortest path.

There are two types of path cost values for PVST+ and Single Spanning Tree: short(16bit value) and long(32bit value), which must be combined for the entire topology. When using ports whose speed is 10 Gbit/s or more, we recommend using long (32-bit value) types. The Switch defaults to short(16bit setting. Multiple Spanning Tree is long(32bit number only. Automatic settings based on Ethernet interface speed differ depending on whether short (16-bit value) or long (32-bit value) types are set.

We recommend lowering the path cost to that of a fast port. The default value of the path cost corresponds to the speed of the port, but can also be changed in the configuration. For the default values according to the speed, refer to the description of the configuration command shown below in "Configuration Command Reference".

For PVST +

spanning-tree pathcost method
spanning-tree vlan pathcost method

For Single Spanning Tree

spanning-tree pathcost method
spanning-tree single pathcost method

Multiple Spanning Tree

Long of spanning-tree pathcost method

(5) Port identifier

Each port in a switch is identified by a parameter called a port ID. Port IDs are used to select a communication path when two or more redundant connections exist between two switches, and the path cost cannot be changed for each port. Note that when redundant connections are used between two switches, we recommend using link aggregation. Use a Spanning Tree Protocol to enable redundant connections between switches that do not support link aggregation.

Port IDs consist of a port priority (4 bits) and a port number (12 bits). The following figure shows a port ID.

Figure 28-3: Port identifier