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Table Of Contents
Understanding Spanning-Tree Features
Spanning-Tree Topology and BPDUs
Bridge ID, Switch Priority, and Extended System ID
Spanning-Tree Interface States
How a Switch or Port Becomes the Root Switch or Root Port
Spanning Tree and Redundant Connectivity
Spanning-Tree Address Management
Accelerated Aging to Retain Connectivity
Spanning-Tree Modes and Protocols
Supported Spanning-Tree Instances
Spanning-Tree Interoperability and Backward Compatibility
Configuring Spanning-Tree Features
Default Spanning-Tree Configuration
Spanning-Tree Configuration Guidelines
Changing the Spanning-Tree Mode.
Configuring a Secondary Root Switch
Configuring the Switch Priority of a VLAN
Configuring Spanning-Tree Timers
Configuring the Forwarding-Delay Time for a VLAN
Configuring the Maximum-Aging Time for a VLAN
Displaying the Spanning-Tree Status
Configuring STP
This chapter describes how to configure the Spanning Tree Protocol (STP) on port-based VLANs on the Cisco ME 3800X and ME 3600X switch. The switch can use the per-VLAN spanning-tree plus (PVST+) protocol based on the IEEE 802.1D standard and Cisco proprietary extensions, or the rapid per-VLAN spanning-tree plus (rapid-PVST+) protocol based on the IEEE 802.1w standard. On the Cisco ME switch, STP is enabled by default on physical interfaces.
For information about the Multiple Spanning Tree Protocol (MSTP) and how to map multiple VLANs to the same spanning-tree instance, see Chapter 15 "Configuring MSTP." For information about other spanning-tree features such as Port Fast, root guard, and so forth, see Chapter 16 "Configuring Optional Spanning-Tree Features."
Note
For complete syntax and usage information for the commands used in this chapter, see the command reference for this release.
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Understanding Spanning-Tree Features
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For configuration information, see the "Configuring Spanning-Tree Features" section.
For information about optional spanning-tree features, see Chapter 16 "Configuring Optional Spanning-Tree Features."
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 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:
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The switch that has all of its ports as the designated role or 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 port priority 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.
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Spanning-Tree Topology and BPDUs
The stable, active spanning-tree topology of a switched network is controlled by these elements:
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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, or on the Cisco ME switch, only through the STP-enabled ports. The BPDUs communicate and compute the spanning-tree topology. Each configuration BPDU contains this information:
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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 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:
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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 14-1.
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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.
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.
The switch supports the IEEE 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 14-1, 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.
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. For more information, see the "Configuring the Root Switch" section, the "Configuring a Secondary Root Switch" section, and the "Configuring the Switch Priority of a VLAN" section.
Spanning-Tree Interface States
Propagation delays can occur when protocol 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 STP port 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:
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A port participating in spanning tree moves through these states:
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Figure 14-1 shows how an interface moves through the states.
Figure 14-1 Spanning-Tree Interface States
When you power up the switch, spanning tree is enabled by default, and every port on the switch, as well as any other port in other switches in the VLAN or network that are participating in spanning tree, 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 spanning-tree interface in the forwarding state, this process occurs:
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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, or to each switch STP port. 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 participating in spanning tree always enters the blocking state after switch initialization.
An interface in the blocking state performs these functions:
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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 should participate in frame forwarding.
An interface in the listening state performs these functions:
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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:
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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:
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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 non operational.
A disabled interface performs these functions:
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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 14-2, 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 might 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 14-2 Spanning-Tree Topology
When the spanning-tree topology is calculated based on default parameters, the path between source and destination end stations in a switched network might 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.
Spanning Tree and Redundant Connectivity
You can create a redundant backbone with spanning tree by connecting two switch interfaces that are participating in spanning tree to another device or to two different devices, as shown in Figure 14-3. Spanning tree automatically disables one interface but enables it if the other one fails. If one link is high-speed and the other is 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 14-3 Spanning Tree and Redundant Connectivity
You can also create redundant links between switches by using EtherChannel groups. For more information, see Chapter 36 "Configuring EtherChannels."
Spanning-Tree Address Management
IEEE 802.1D specifies 17 multicast addresses, ranging from 0x00180C2000000 to 0x0180C2000010, to be used by different bridge protocols. These addresses are static addresses that cannot be removed.
Regardless of the spanning-tree state, each switch receives but does not forward packets destined for addresses between 0x0180C2000000 and 0x0180C200000F.
If spanning tree is enabled, the CPU on the switch receives packets destined for 0x0180C2000000 and 0x0180C2000010. If spanning tree is disabled, the switch forwards those packets as unknown multicast addresses.
Accelerated Aging to Retain Connectivity
The default for aging dynamic addresses is 5 minutes, the default setting of the mac address-table aging-time global configuration command. However, a spanning-tree reconfiguration can cause many station locations to change. Because these stations could be unreachable for 5 minutes or more during a reconfiguration, the address-aging time is accelerated so that station addresses can be dropped from the address table and then relearned. The accelerated aging is the same as the forward-delay parameter value (spanning-tree vlan vlan-id forward-time seconds global configuration command) when the spanning tree reconfigures.
Because each VLAN is a separate spanning-tree instance, the switch accelerates aging on a per-VLAN basis. A spanning-tree reconfiguration on one VLAN can cause the dynamic addresses learned on that VLAN to be subject to accelerated aging. Dynamic addresses on other VLANs can be unaffected and remain subject to the aging interval entered for the switch.
Spanning-Tree Modes and Protocols
The switch ports support these spanning-tree modes and protocols:
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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 that no one link is 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.
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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.
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The most common initial deployment of MSTP is in the backbone and distribution layers of a Layer 2 switched network. For more information, see Chapter 15 "Configuring MSTP."
For information about the number of supported spanning-tree instances, see the next section.
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 65 MST instances. The number of VLANs that can be mapped to a particular MST instance is unlimited.
Spanning-Tree Interoperability and Backward Compatibility
Table 14-2 lists the interoperability and compatibility among the supported spanning-tree modes in a network.
In a mixed MSTP and PVST+ network, the common spanning-tree (CST) root must be inside the MST backbone, and a PVST+ switch cannot 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 should be at the edge of the network.
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 IEEE 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 IEEE 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 IEEE 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 is 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 IEEE 802.1Q trunks, and no user configuration is required. The external spanning-tree behavior on access ports is not affected by PVST+.
For more information on IEEE 802.1Q trunks, see Chapter 11 "Configuring VLANs."
Configuring Spanning-Tree Features
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Default Spanning-Tree Configuration
Table 14-3 shows the default spanning-tree configuration.
Spanning-Tree Configuration Guidelines
If more VLANs are defined than there are spanning-tree instances, you can enable PVST+ or rapid PVST+ on STP ports s in only 128 VLANs on the switch. The remaining VLANs operate with spanning tree disabled. However, you can map multiple VLANs to the same spanning-tree instances by using MSTP. For more information, see Chapter 15 "Configuring MSTP."
If 128 instances of spanning tree are already in use, you can disable spanning tree on STP ports in one of the VLANs and then enable it on the VLAN where you want it to run. Use the no spanning-tree vlan vlan-id global configuration command to disable spanning tree on a specific VLAN, and use the spanning-tree vlan vlan-id global configuration command to enable spanning tree on the desired VLAN.
Caution
If you have already used all available spanning-tree instances on your switch, adding another VLAN creates a VLAN that is not running spanning tree on that switch. If you have the default allowed list on the trunk ports of that switch, the new VLAN is carried on all trunk ports. Depending on the topology of the network, this could create a loop in the new VLAN that will not be broken, particularly if there are several adjacent switches that have all run out of spanning-tree instances. You can prevent this possibility by setting up allowed lists on the trunk ports of switches that have used up their allocation of spanning-tree instances. Setting up allowed lists is not necessary in many cases and can make it more labor-intensive to add another VLAN to the network.
Spanning-tree commands control the configuration of VLAN spanning-tree instances. You create a spanning-tree instance when you assign an STP port (an NNI or ENI with STP enabled) to a VLAN. The spanning-tree instance is removed when the last port is moved to another VLAN. You can configure switch and port parameters before a spanning-tree instance is created; these parameters are applied when the spanning-tree instance is created.
The switch supports PVST+, rapid PVST+, and MSTP, but only one version can be active at any time. (For example, all VLANs run PVST+, all VLANs run rapid PVST+, or all VLANs run MSTP.) For information about the different spanning-tree modes and how they interoperate, see the "Spanning-Tree Interoperability and Backward Compatibility" section.
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Changing the Spanning-Tree Mode.
The switch supports three spanning-tree modes: PVST+, rapid PVST+, or MSTP. By default, the switch runs the rapid PVST+ protocol on all NNIs and ENIs on which spanning tree is enabled.
Beginning in privileged EXEC mode, follow these steps to change the spanning-tree mode. If you want to enable a mode that is different from the default mode, this procedure is required.
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configure terminal
Enter global configuration mode.
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spanning-tree mode {pvst | mst | rapid-pvst}
Configure a spanning-tree mode on STP ports on the switch.
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interface interface-id
(Recommended only for rapid-PVST+ mode) Specify an STP port to configure, and enter interface configuration mode. Valid interfaces include physical ports, VLANs, and port channels. The VLAN ID range is 1 to 4094. The port-channel range is 1 to 26.
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spanning-tree link-type point-to-point
(Recommended only for rapid-PVST+ mode) Specify that the link type for this port is point-to-point.
If you connect this port to a remote port through a point-to-point link and the local port becomes a designated port, the switch negotiates with the remote port and rapidly changes the local port to the forwarding state.
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end
Return to privileged EXEC mode.
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clear spanning-tree detected-protocols
(Recommended only for rapid-PVST+ mode) If any port on the switch running spanning tree is connected to a port on a legacy IEEE 802.1D switch, restart the protocol migration process on the entire switch.
This step is optional if the designated switch detects that this switch is running rapid PVST+.
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show spanning-tree summary
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show spanning-tree interface interface-id
Verify your entries.
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copy running-config startup-config
(Optional) Save your entries in the configuration file.
To return to the default setting, use the no spanning-tree mode global configuration command. To return the port to its default spanning-tree mode setting, use the no spanning-tree link-type interface configuration command.
Disabling Spanning Tree
Spanning tree is enabled by default on all ports in VLAN 1 and in all newly created VLANs up to the spanning-tree limit specified in the "Supported Spanning-Tree Instances" section. Disable spanning tree only if you are sure there are no loops in the network topology.
Caution
Beginning in privileged EXEC mode, follow these steps to disable spanning-tree on a per-VLAN basis. This procedure is optional.
To re-enable spanning-tree, use the spanning-tree vlan vlan-id global configuration command.
Configuring the Root Switch
The switch maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge ID, consisting of the switch priority and the switch MAC address, is associated with each instance. For each VLAN, the switch with the lowest bridge ID becomes the root switch for that VLAN.
To configure a switch to become the root for the specified VLAN, use the spanning-tree vlan vlan-id root global configuration command to modify the switch priority from the default value (32768) to a significantly lower value. When you enter this command, the software checks the switch priority of the root switches for each VLAN. Because of the extended system ID support, the switch sets its own priority for the specified VLAN to 24576 if this value will cause this switch to become the root for the specified VLAN.
If any root switch for the specified VLAN has a switch priority lower than 24576, the switch sets its own priority for the specified VLAN to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit switch priority value as shown in Table 14-1.)
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If your network consists of switches that both do and do not support the extended system ID, it is unlikely that the switch with the extended system ID support will become the root switch. The extended system ID increases the switch priority value every time the VLAN number is greater than the priority of the connected switches running older software.
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Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.
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Beginning in privileged EXEC mode, follow these steps to configure a switch to become the root for the specified VLAN. This procedure is optional.
To return to the default setting, use the no spanning-tree vlan vlan-id root global configuration command.
Configuring a Secondary Root Switch
When you configure a switch as the secondary root, the switch priority is modified from the default value (32768) to 28672. The switch is then likely to become the root switch for the specified VLAN if the primary root switch fails. This is assuming that the other network switches use the default switch priority of 32768 and therefore are unlikely to become the root switch.
You can execute this command on more than one switch to configure multiple backup root switches. Use the same network diameter and hello-time values that you used when you configured the primary root switch with the spanning-tree vlan vlan-id root primary global configuration command.
Beginning in privileged EXEC mode, follow these steps to configure a switch to become the secondary root for the specified VLAN. This procedure is optional.
Step 1
configure terminal
Enter global configuration mode.
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spanning-tree vlan vlan-id root secondary [diameter net-diameter [hello-time seconds]]
Configure a switch to become the secondary root for the specified VLAN.
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Use the same network diameter and hello-time values that you used when configuring the primary root switch. See the "Configuring the Root Switch" section.
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end
Return to privileged EXEC mode.
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show spanning-tree detail
Verify your entries.
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copy running-config startup-config
(Optional) Save your entries in the configuration file.
To return to the default setting, use the no spanning-tree vlan vlan-id root global configuration command.
Configuring Port Priority
If a loop occurs, spanning tree uses the port priority when selecting a spanning-tree port to put into the forwarding state. You can assign higher priority values (lower numerical values) to ports that you want selected first and lower priority values (higher numerical values) to ones that you want selected last. If all spanning-tree ports have the same priority value, spanning tree puts the port with the lowest interface number in the forwarding state and blocks the other interfaces.
Beginning in privileged EXEC mode, follow these steps to configure the port priority of a spanning-tree port. This procedure is optional.
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To return to the default spanning-tree setting, use the no spanning-tree [vlan vlan-id] port-priority interface configuration command. For information on how to configure load sharing on trunk ports by using spanning-tree port priorities, see the "Configuring Trunk Ports for Load Sharing" section.
Configuring Path Cost
The spanning-tree path cost default value is derived from the media speed of an interface (port running spanning tree or port channel of multiple ports running spanning tree). If a loop occurs, spanning tree uses cost when selecting an interface to put in the forwarding state. You can assign lower cost values to interfaces that you want selected first and higher cost values that you want selected last. If all NNIs (or port channels) have the same cost value, spanning tree puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.
Beginning in privileged EXEC mode, follow these steps to configure the cost of an interface. This procedure is optional.
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To return to the default setting, use the no spanning-tree [vlan vlan-id] cost interface configuration command. For information on how to configure load sharing on trunk ports by using spanning-tree path costs, see the "Configuring Trunk Ports for Load Sharing" section.
Configuring the Switch Priority of a VLAN
You can configure the switch priority and make it more likely that the switch will be chosen as the root switch.
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Beginning in privileged EXEC mode, follow these steps to configure the switch priority of a VLAN. This procedure is optional.
To return to the default setting, use the no spanning-tree vlan vlan-id priority global configuration command.
Configuring Spanning-Tree Timers
Configuring the Hello Time
You can configure the interval between the generation of configuration messages by the root switch by changing the hello time.
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Beginning in privileged EXEC mode, follow these steps to configure the hello time of a VLAN. This procedure is optional.
To return to the default setting, use the no spanning-tree vlan vlan-id hello-time global configuration command.
Configuring the Forwarding-Delay Time for a VLAN
Beginning in privileged EXEC mode, follow these steps to configure the forwarding-delay time for a VLAN. This procedure is optional.
To return to the default setting, use the no spanning-tree vlan vlan-id forward-time global configuration command.
Configuring the Maximum-Aging Time for a VLAN
Beginning in privileged EXEC mode, follow these steps to configure the maximum-aging time for a VLAN. This procedure is optional.
To return to the default setting, use the no spanning-tree vlan vlan-id max-age global configuration command.
Displaying the Spanning-Tree Status
To display the spanning-tree status, use one or more of the privileged EXEC commands in Table 14-5:
You can clear spanning-tree counters by using the clear spanning-tree [interface interface-id] privileged EXEC command.
For information about other keywords for the show spanning-tree privileged EXEC command, see the command reference for this release.
