Prepare to Configure Spanning Tree Protocol (STP) on a Catalyst Switch

Introduction      STP Overview      Spanning-Tree Topology and BPDUs      Bridge ID, Switch Priority, and Extended System ID      Spanning-Tree Interface States           Blocking State           Listening State           Learning State           Forwarding State           Disabled State      How a Switch or Port Becomes the Root Switch or Root Port      Spanning Tree and Redundant Connectivity      Spanning-Tree Modes and Protocols      Supported Spanning-Tree Instances      Spanning-Tree Interoperability and Backward Compatibility      STP and IEEE 802.1Q Trunks      Next Step      Related Information

Download PDF   Prepare to Configure Spanning Tree Protocol (STP) on a Catalyst Switch

Introduction

This document provides general information to understand Spanning Tree Protocol (STP) feature, the terminology, different modes and protocols along with the guidelines that assists you to prepare, to configure Spanning Tree Protocol on a Catalyst Switch.

If you are already aware of the STP features, its concepts and terminology and need to configure STP on a Catalyst switch refer to the documents Configure Spanning Tree Protocol (STP) on a Catalyst Switch that runs Cisco IOS Software or Configure Spanning Tree Protocol (STP) on a Catalyst Switch that runs Catalyst OS Software.

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STP Overview

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 could not 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 port in the spanning tree

• Backup—A blocked port in a loopback configuration

  Switches that have ports with the roles assigned under STP Overview are called root or designated switches.

  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, root port for the switched network, the root port and designated port for each switched segment.

  When two ports on a switch are a part of a loop, the spanning-tree port priority and path cost settings controls the port that is put in the forwarding state and that 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.

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Spanning-Tree Topology and BPDUs

The stable, active spanning-tree topology of a switched network is controlled by these elements:

1. The unique bridge ID (switch priority and MAC address) associated with each VLAN on each switch.

2. The spanning-tree path cost to the root switch.

3. The port identifier (port priority and MAC address) associated with each Layer 2 interface.

   When the switches in a network are powered up, each functions as the root switch. Each switch sends a configuration BPDU through all of its ports. The BPDUs communicate and compute the spanning-tree topology. Each configuration BPDU contains this information:

1. The unique bridge ID of the switch that the sending switch identifies as the root switch

2. The spanning-tree path cost to the root

3. The bridge ID of the sending switch

4. Message age

5. The identifier of the sending interface

6. Values for the hello, forward delay, and max-age protocol timers

When a switch receives a configuration BPDU that contains superior information (lower bridge ID, lower path cost, and so forth), it stores the information for that port. If this BPDU is received on the root port of the switch, the switch also forwards it with an updated BPDU message to all attached LANs for which it is the designated switch.

If a switch receives a configuration BPDU that contains inferior information to that currently stored for that port, it discards the BPDU. If the switch is a designated switch for the LAN from which the inferior BPDU was received, it sends that LAN a BPDU containing the up-to-date information stored for that port. In this way, inferior information is discarded, and superior information is propagated on the network.

A BPDU exchange results in these actions:

One switch in the network is elected as the root switch (the logical center of the spanning-tree topology in a switched network). For each VLAN, the switch with the highest switch priority (the lowest numerical priority value) is elected as the root switch. If all switches are configured with the default priority (32768), the switch with the lowest MAC address in the VLAN becomes the root switch. The switch priority value occupies the most significant bits of the bridge ID, as shown in Table 16-1.

1. The shortest distance to the root switch is calculated for each switch based on the path cost.

2. A root port is selected for each switch (except the root switch). This port provides the best path (lowest cost) when the switch forwards packets to the root switch.

3. A designated switch for each LAN segment is selected. The designated switch incurs the lowest path cost when forwarding packets from that LAN to the root switch. The port through which the designated switch is attached to the LAN is called the designated port.

4. All paths that are not needed to reach the root switch from anywhere in the switched network are placed in the spanning-tree blocking mode.

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Bridge ID, Switch Priority, and Extended System ID

The IEEE 802.1D standard requires that each switch has an unique bridge identifier (bridge ID), which controls the selection of the root switch. Because, each VLAN is considered as a different logical bridge with PVST+ and rapid PVST+, the same switch must have as many different bridge IDs as VLANs configured on it. Each VLAN on the switch has a unique 8-byte bridge ID. The two most-significant bytes are used for the switch priority, and the remaining six bytes are derived from the switch MAC address.

Few switches support the 802.1t spanning-tree extensions, and some of the bits previously used for the switch priority are now used as the VLAN identifier. The result is that, fewer MAC addresses are reserved for the switch, and a larger range of VLAN IDs can be supported, all while maintaining the uniqueness of the bridge ID. As shown in Table 16-1, the two bytes previously used for the switch priority are reallocated into a 4-bit priority value and a 12-bit extended system ID value equal to the VLAN ID.

Table 16-1 Switch Priority Value and Extended System ID
Switch Priority Value				Extended System ID (Set Equal to the VLAN ID)
Bit 16	Bit 15	Bit 14	Bit 13	Bit 12	Bit 11	Bit 10	Bit 9	Bit 8	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1
32768	16384	8192	4096	2048	1024	512	256	128	64	32	16	8	4	2	1

Spanning tree uses the extended system ID, the switch priority, and the allocated spanning-tree MAC address to make the bridge ID unique for each VLAN.

Support for the extended system ID affects how you manually configure the root switch, the secondary root switch, and the switch priority of a VLAN. For example, when you change the switch priority value, you change the probability that the switch will be elected as the root switch. Configuring a higher value decreases the probability; a lower value increases the probability.

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Spanning-Tree Interface States

Propagation delays can occur when STP information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When an interface transitions directly from nonparticipation in the spanning-tree topology to the forwarding state, it can create temporary data loops. Interfaces must wait for new topology information to propagate through the switched LAN before starting to forward frames. They must allow the frame lifetime to expire for forwarded frames that have used the old topology.

Each Layer 2 interface on a switch using spanning tree exists in one of these states:

• Blocking—The interface does not participate in frame forwarding.

• Listening—The first transitional state after the blocking state when the spanning tree decides that the interface must participate in frame forwarding

• Learning—The interface prepares to participate in frame forwarding.

• Forwarding—The interface forwards frames.

• Disabled—The interface does not participate in spanning tree because of a shutdown port, no link on the port, or no spanning-tree instance running on the port.

  An interface moves through these states:

• From initialization to blocking

• From listening to learning or to disabled

• From learning to forwarding or to disabled

• From forwarding to disabled

  Figure 16-1 illustrates how an interface moves through the states.

  

When you power up the switch, spanning tree is enabled by default, and every interface in the switch, VLAN, or network goes through the blocking state and the transitory states of listening and learning. Spanning tree stabilizes each interface at the forwarding or blocking state.

When the spanning-tree algorithm places a Layer 2 interface in the forwarding state, this process occurs:

1. The interface is in the listening state while spanning tree waits for protocol information to transition the interface to the blocking state.

2. While spanning tree waits, the forward-delay timer to expire, it moves the interface to the learning state and resets the forward-delay timer.

3. In the learning state, the interface continues to block frame forwarding as the switch learns end-station location information for the forwarding database.

4. When the forward-delay timer expires, spanning tree moves the interface to the forwarding state, where both learning and frame forwarding are enabled.

Blocking State

A Layer 2 interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU is sent to each switch interface. A switch initially functions as the root until it exchanges BPDUs with other switches. This exchange establishes which switch in the network is the root or root switch. If there is only one switch in the network, no exchange occurs, the forward-delay timer expires, and the interface moves to the listening state. An interface always enters the blocking state after switch initialization.

An interface in the blocking state performs these functions:

• Discards frames received on the interface

• Discards frames switched from another interface for forwarding

• Does not learn addresses

• Receives BPDUs

Listening State

The listening state is the first state a Layer 2 interface enters after the blocking state. The interface enters this state when the spanning tree decides that the interface must participate in frame forwarding.

An interface in the listening state performs these functions:

• Discards frames received on the interface

• Discards frames switched from another interface for forwarding

• Does not learn addresses

• Receives BPDUs

Learning State

A Layer 2 interface in the learning state prepares to participate in frame forwarding. The interface enters the learning state from the listening state.

An interface in the learning state performs these functions:

• Discards frames received on the interface

• Discards frames switched from another interface for forwarding

• Learns addresses

• Receives BPDUs

Forwarding State

A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwarding state from the learning state.

An interface in the forwarding state performs these functions:

• Receives and forwards frames received on the interface

• Forwards frames switched from another interface

• Learns addresses

• Receives BPDUs

Disabled State

A Layer 2 interface in the disabled state does not participate in frame forwarding or in the spanning tree. An interface in the disabled state is nonoperational.

A disabled interface performs these functions:

• Discards frames received on the interface

• Discards frames switched from another interface for forwarding

• Does not learn addresses

• Does not receive BPDUs

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How a Switch or Port Becomes the Root Switch or Root Port

If all switches in a network are enabled with default spanning-tree settings, the switch with the lowest MAC address becomes the root switch. In Figure 16-2, Switch A is elected as the root switch because the switch priority of all the switches is set to the default (32768) and Switch A has the lowest MAC address. However, because of traffic patterns, number of forwarding interfaces, or link types, Switch A would not be the ideal root switch. By increasing the priority (lowering the numerical value) of the ideal switch so that it becomes the root switch, you force a spanning-tree recalculation to form a new topology with the ideal switch as the root.

Figure 16-2 illustrates how a switch or port becomes the root switch or root port

When the spanning-tree topology is calculated based on default parameters, the path between source and destination end stations in a switched network would not be ideal. For instance, connecting higher-speed links to an interface that has a higher number than the root port can cause a root-port change. The goal is to make the fastest link the root port.

For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on Switch B (a 10/100 link) is the root port. Network traffic might be more efficient over the Gigabit Ethernet link. By changing the spanning-tree port priority on the Gigabit Ethernet port to a higher priority (lower numerical value) than the root port, the Gigabit Ethernet port becomes the new root port.

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Spanning Tree and Redundant Connectivity

You can create a redundant backbone with spanning tree by connecting two switch interfaces to another device or to two different devices, as shown in Figure 16-3. Spanning tree automatically disables one interface but enables it if the other one fails. If one link is of high-speed and the other is of low-speed, the low-speed link is always disabled. If the speeds are the same, the port priority and port ID are added together, and spanning tree disables the link with the lowest value.

Figure 16-2 illustrates how to create a redundant backbone with spanning tree.

You can also create redundant links between switches by using EtherChannel groups. For more information, refer to Configure EtherChannel on a Cisco Catalyst Switch with Cisco Network Assistant.

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Spanning-Tree Modes and Protocols

The switch supports these spanning-tree modes and protocols:

• PVST+—This spanning-tree mode is based on the IEEE 802.1D standard and Cisco proprietary extensions. It is the default spanning-tree mode used on all Ethernet, Fast Ethernet, and Gigabit Ethernet port-based VLANs. The PVST+ runs on each VLAN on the switch up to the maximum supported, ensuring that each has a loop-free path through the network.

  The PVST+ provides Layer 2 load balancing for the VLAN on which it runs. You can create different logical topologies by using the VLANs on your network to ensure that all of your links are used but none of them are oversubscribed. Each instance of PVST+ on a VLAN has a single root switch. This root switch propagates the spanning-tree information associated with that VLAN to all other switches in the network. Because, each switch has the same information about the network, this process ensures that the network topology is maintained.

• Rapid PVST+—This spanning-tree mode is the same as PVST+ except that it uses a rapid convergence based on the IEEE 802.1w standard. To provide rapid convergence, the rapid PVST+ immediately deletes dynamically learned MAC address entries on a per-port basis upon receiving a topology change. By contrast, PVST+ uses a short aging time for dynamically learned MAC address entries.

  The rapid PVST+ uses the same configuration as PVST+ (except where noted), and the switch needs only minimal extra configuration. The benefit of rapid PVST+ is that, you can migrate a large PVST+ install base to rapid PVST+ without having to learn the complexities of the MSTP configuration and without having to reprovision your network. In rapid-PVST+ mode, each VLAN runs its own spanning-tree instance up to the maximum supported.

• MSTP—This spanning-tree mode is based on the IEEE 802.1s standard. You can map multiple VLANs to the same spanning-tree instance, which reduces the number of spanning-tree instances required to support a large number of VLANs. The MSTP runs on top of the RSTP (based on IEEE 802.1w), which provides for rapid convergence of the spanning tree by eliminating the forward delay and by quickly transitioning root ports and designated ports to the forwarding state. You cannot run MSTP without RSTP.

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Supported Spanning-Tree Instances

In PVST+ or rapid-PVST+ mode, the switch supports up to 128 spanning-tree instances.

In MSTP mode, the switch supports up to 16 MST instances. The number of VLANs that can be mapped to a particular MST instance is unlimited.

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Spanning-Tree Interoperability and Backward Compatibility

Table 16-2 lists the interoperability and compatibility among the supported spanning-tree modes in a network.

Table 16-2 PVST+, MSTP, and Rapid-PVST+ Interoperability
	PVST+	MSTP	Rapid PVST+
PVST+	Yes	Yes (with restrictions)	Yes (reverts to PVST+)
MSTP	Yes (with restrictions)	Yes	Yes (reverts to PVST+)
Rapid PVST+	Yes (reverts to PVST+)	Yes (reverts to PVST+)	Yes

In a mixed MSTP and PVST+ network, the common spanning-tree (CST) root must be inside the MST backbone, and a PVST+ switch is unable to connect to multiple MST regions.

When a network contains switches running rapid PVST+ and switches running PVST+, we recommend that the rapid-PVST+ switches and PVST+ switches be configured for different spanning-tree instances. In the rapid-PVST+ spanning-tree instances, the root switch must be a rapid-PVST+ switch. In the PVST+ instances, the root switch must be a PVST+ switch. The PVST+ switches must be at the edge of the network.

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STP and IEEE 802.1Q Trunks

The IEEE 802.1Q standard for VLAN trunks imposes some limitations on the spanning-tree strategy for a network. The standard requires only one spanning-tree instance for all VLANs allowed on the trunks. However, in a network of Cisco switches connected through 802.1Q trunks, the switches maintain one spanning-tree instance for each VLAN allowed on the trunks.

When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch uses PVST+ to provide spanning-tree interoperability. If rapid PVST+ is enabled, the switch uses it instead of PVST+. The switch combines the spanning-tree instance of the 802.1Q VLAN of the trunk with the spanning-tree instance of the non-Cisco 802.1Q switch.

However, all PVST+ or rapid-PVST+ information are maintained by Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a single trunk link between the switches.

PVST+ is automatically enabled on 802.1Q trunks, and no user configuration is required. The external spanning-tree behavior on access ports and Inter-Switch Link (ISL) trunk ports is not affected by PVST+.

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Next Step

You have completed this procedure and prepared to configure STP.

To make other changes to your switch, refer to the Switch Support Page.

To configure other devices in your network, refer to the Configuration Overview Page.

If this information does not solve your problem, contact the SMB Technical Assistance Center (SMB TAC) for assistance.

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Related Information

• Basic Setup and Configuration for the Catalyst Switch
• Configure Spanning Tree Protocol (STP) on a Catalyst Switch that runs Cisco IOS Software
• Configure Spanning Tree Protocol (STP) on a Catalyst Switch that runs Catalyst OS Software