Table Of Contents
Creating the Spanning Tree Topology
Proposal and Agreement Handshake
Detecting Unidirectional Link Failure
Rapid PVST+ and IEEE 802.1Q Trunks
Rapid PVST+ Interoperation with Legacy 802.1D STP
Rapid PVST+ Interoperation with 802.1s MST
Licensing Requirements for Rapid PVST+
Prerequisites for Configuring Rapid PVST+
Setting All STP Parameters on the Device to Default Values
Disabling or Enabling Rapid PVST+ per VLAN
Configuring the Primary and Secondary Roots and the Switch Priority
Configuring the Rapid PVST+ Port Priority
Configuring the Rapid PVST+ Path-Cost Method and Port Cost
Setting All Rapid PVST+ Parameters to Default Values per Interface
Configuring the Rapid PVST+ Hello Time for a VLAN
Configuring the Rapid PVST+ Forward Delay Time for a VLAN
Configuring the Rapid PVST+ Maximum Age Time for a VLAN
Field Descriptions for Rapid PVST+
Device View: VLAN: Details: VLAN Setting Section
Device View: VLAN: Details: Port Setting Section
Network View: Device: Details: VLAN Setting Section
Network View: Device: Details: Port Setting Section
Configuring Rapid PVST+
This chapter describes how to configure the Rapid per VLAN Spanning Tree (Rapid PVST+) protocol on NX-OS devices.
For more information about the Data Center Network Manager features, see the Cisco DCNM Fundamentals Configuration Guide.
This chapter includes the following sections:
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Information About Rapid PVST+
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Information About Rapid PVST+
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--logging-level spanning-tree 6
--logging logfile messages 6
--logging event link-status default
See the Cisco NX-OS System Management Configuration Guide for information on logging levels.
Note
The Spanning Tree Protocol (STP) was implemented to provide a loop-free network at Layer 2 of the network. Rapid PVST+ is an updated implementation of STP that allows you to create one spanning tree topology for each VLAN. Rapid PVST+ is the default STP mode on the device.
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This section describes the Rapid PVST+ protocol, which is the IEEE 802.1w standard, Rapid Spanning Tree Protocol (RSTP), implemented on a per VLAN basis. Rapid PVST+ interoperates with the IEEE 802.1Q VLAN standard, which mandates a single STP instance for all VLANs, rather than per-VLAN. (See the "Rapid PVST+ and IEEE 802.1Q Trunks" section).
Rapid PVST+ is enabled by default on the default VLAN (VLAN1) and on all newly created VLANs on the device. Rapid PVST+ interoperates with devices that run legacy IEEE 802.1D STP (see the "Rapid PVST+ Interoperation with Legacy 802.1D STP" section).
RSTP is an improvement on the original STP standard, 802.1D, which allows faster convergence.
This section includes an overview of Rapid PVST+ and consists of these topics:
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STP
STP is a Layer 2 link-management protocol that provides path redundancy while preventing loops in the network.
This section provides a basic understanding of STP in the following topics:
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Overview
In order for a Layer 2 Ethernet network to function properly, only one active path can exist between any two stations. STP 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.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a network. The STP algorithm calculates the best loop-free path throughout a switched Layer 2 network. Layer 2 LAN ports send and receive STP frames, which are called Bridge Protocol Data Units (BPDUs), at regular intervals. Network devices do not forward these frames, but use the frames to construct a loop-free path.
Multiple active paths between end stations cause loops in the network. If a loop exists in the network, end stations might receive duplicate messages and network devices might learn end station MAC addresses on multiple Layer 2 LAN ports.
STP defines a tree with a root bridge and a loop-free path from the root to all network devices in the Layer 2 network. STP forces redundant data paths into a blocked state. If a network segment in the spanning tree fails and a redundant path exists, the STP algorithm recalculates the spanning tree topology and activates the blocked path.
When two Layer 2 LAN ports on a network device are part of a loop, the STP port priority and port path-cost setting determine which port on the device is put in the forwarding state and which port is put in the blocking state. The STP port priority value is the efficiency with which that location allows the port to pass traffic. The STP port path-cost value is derived from the media speed.
How a Topology is Created
All devices in a LAN that participate in a spanning tree gather information about other switches in the network by exchanging BPDUs. This exchange of BPDUs results in the following actions:
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The topology on an active switched network is determined by the following:
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In a switched network, the root switch is the logical center of the spanning tree topology. STP uses BPDUs to elect the root switch and root port for the switched network.
Bridge ID
Each VLAN on each network device has a unique 64-bit bridge ID that consists of a bridge priority value, an extended system ID (IEEE 802.1t), and an STP MAC address allocation.
This section includes the following topics:
Bridge Priority Value
The bridge priority is a 4-bit value when the extended system ID is enabled.
(See the "Configuring the Rapid PVST+ Hello Time for a VLAN" section.) You can only specify a device bridge ID (used by the spanning tree algorithm to determine the identity of the root bridge; the lowest number is preferred) as a multiple of 4096.
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Extended System ID
A 12-bit extended system ID field is part of the bridge ID (see Figure 4-1).
Figure 4-1 Bridge ID with Extended System ID
The device always uses the 12-bit extended system ID.
Combined with the bridge ID, the system ID extension functions as the unique identifier for a VLAN (see Table 4-1).
STP MAC Address Allocation
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Because MAC address reduction is always enabled on the device, you should also enable MAC address reduction on all other Layer 2 connected network devices to avoid undesirable root bridge election and spanning tree topology issues.
When MAC address reduction is enabled, the root bridge priority becomes a multiple of 4096 plus the VLAN ID. You can only specify a device bridge ID (used by the spanning tree algorithm to determine the identity of the root bridge; the lowest number is preferred) as a multiple of 4096. Only the following values are possible:
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STP uses the extended system ID plus a MAC address to make the bridge ID unique for each VLAN.
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BPDUs
Network devices transmit BPDUs throughout the STP instance. Each network device sends configuration BPDUs to communicate and compute the spanning tree topology. Each configuration BPDU contains the following minimal information:
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When a network device transmits a Rapid PVST+ BPDU frame, all network devices connected to the VLAN on which the frame is transmitted receive the BPDU. When a network device receives a BPDU, it does not forward the frame but instead uses the information in the frame to calculate a BPDU. If the topology changes, the device initiates a BPDU exchange.
A BPDU exchange results in the following:
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See the "Rapid PVST+ BPDUs" section for information about the fields that Rapid PVST+ adds to the BPDU.
Election of the Root Bridge
For each VLAN, the network device with the highest bridge ID (that is, the lowest numerical ID value) is elected as the root bridge. If all network devices are configured with the default priority (32768), the network device with the lowest MAC address in the VLAN becomes the root bridge. The bridge priority value occupies the most significant bits of the bridge ID.
When you change the bridge priority value, you change the probability that the device will be elected as the root bridge. Configuring a lower value increases the probability; a higher value decreases the probability.
The STP root bridge is the logical center of each spanning tree topology in a Layer 2 network. All paths that are not needed to reach the root bridge from anywhere in the Layer 2 network are placed in STP blocking mode.
BPDUs contain information about the transmitting bridge and its ports, including bridge and MAC addresses, bridge priority, port priority, and path cost. STP uses this information to elect the root bridge for the STP instance, to elect the root port that leads to the root bridge, and to determine the designated port for each Layer 2 segment.
Creating the Spanning Tree Topology
In Figure 4-2, Switch A is elected as the root bridge because the bridge priority of all the network devices is set to the default (32768) and Switch A has the lowest MAC address. However, due to traffic patterns, the number of forwarding ports, or link types, Switch A might not be the ideal root bridge. By increasing the priority (lowering the numerical value) of the ideal network device so that it becomes the root bridge, you force an STP recalculation to form a new spanning tree topology with the ideal network device as the root.
Figure 4-2 Spanning Tree Topology
When the spanning tree topology is calculated based on default parameters, the path between the source and destination end stations in a switched network might not be ideal. For instance, connecting higher-speed links to a port that has a higher number than the current 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 fiber-optic link, and another port on Switch B (an unshielded twisted-pair [UTP] link) is the root port. Network traffic might be more efficient over the high-speed fiber-optic link. By changing the STP port priority on the fiber-optic port to a higher priority (lower numerical value) than the root port, the fiber-optic port becomes the new root port.
Rapid PVST+
This section includes the following topics:
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Overview
Rapid PVST+ is the IEEE 802.1w (RSTP) standard implemented per VLAN. A single instance of STP runs on each configured VLAN (if you do not manually disable STP). Each Rapid PVST+ instance on a VLAN has a single root switch. You can enable and disable STP on a per-VLAN basis when you are running Rapid PVST+.
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Rapid PVST+ uses point-to-point wiring to provide rapid convergence of the spanning tree. The spanning tree reconfiguration can occur in less than 1 second with Rapid PVST+ (in contrast to 50 seconds with the default settings in the 802.1D STP). The device automatically checks the PVID.
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Using Rapid PVST+, STP convergence occurs rapidly. By default, each designated port in the STP sends out a BPDU every 2 seconds. On a designated port in the topology, if hello messages are missed three consecutive times, or if the maximum age expires, the port immediately flushes all protocol information in the table. A port considers that it loses connectivity to its direct neighbor designated port if it misses three BPDUs or if the maximum age expires. This rapid aging of the protocol information allows quick failure detection.
Rapid PVST+ provides for rapid recovery of connectivity following the failure of a device, a device port, or a LAN. It provides rapid convergence for edge ports, new root ports, and ports connected through point-to-point links as follows:
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Rapid PVST+ achieves rapid transition to the forwarding state only on edge ports and point-to-point links. Although the link type is configurable, the system automatically derives the link type information from the duplex setting of the port. Full-duplex ports are assumed to be point-to-point ports, while half-duplex ports are assumed to be shared ports.
Edge ports do not generate topology changes, but all other designated and root ports generate a topology change (TC) BPDU when they either fail to receive three consecutive BPDUs from the directly connected neighbor or the maximum age times out. At this point, the designated or root port sends a BPDU with the TC flag set. The BPDUs continue to set the TC flag as long as the TC While timer runs on that port. The value of the TC While timer is the value set for the hello time plus 1 second. The initial detector of the topology change immediately floods this information throughout the entire topology.
When Rapid PVST+ detects a topology change, the protocol does the following:
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The topology change notification floods quickly across the entire topology. The system flushes dynamic entries immediately on a per-port basis when it receives a topology change.
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The proposal and agreement sequence then quickly propagates toward the edge of the network and quickly restores connectivity after a topology change (see the "Synchronization of Port Roles" section).
Rapid PVST+ BPDUs
Rapid PVST+ and 802.1w use all six bits of the flag byte to add the following:
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Figure 4-3 shows the use of the BPDU flags in Rapid PVST+,
Figure 4-3 Rapid PVST+ Flag Byte in BPDU
Another important change is that the Rapid PVST+ BPDU is type 2, version 2, which makes it possible for the device to detect connected legacy (802.1D) bridges. The BPDU for 802.1D is type 0, version 0.
Proposal and Agreement Handshake
As shown in Figure 4-4, switch A is connected to switch B through a point-to-point link, and all of the ports are in the blocking state. Assume that the priority of switch A is a smaller numerical value than the priority of switch B.
Figure 4-4 Proposal and Agreement Handshaking for Rapid Convergence
Device A sends a proposal message (a configuration BPDU with the proposal flag set) to device B, proposing itself as the designated device (see Figure 4-4).
After receiving the proposal message, device B selects as its new root port the port from which the proposal message was received, forces all nonedge ports to the blocking state, and sends an agreement message (a BPDU with the agreement flag set) through its new root port.
After receiving the agreement message from switch B, device A also immediately transitions its designated port to the forwarding state. No loops in the network can form because device B blocked all of its nonedge ports and because there is a point-to-point link between switches A and B. (See the "Port States" section for information on port states.)
When device C connects to device B, a similar set of handshaking messages are exchanged. Device C selects the port connected to device B as its root port, and both ends of the link immediately transition to the forwarding state. With each iteration of this handshaking process, one more device joins the active topology. As the network converges, this proposal-agreement handshaking progresses from the root toward the leaves of the spanning tree.
The switch learns the link type from the port duplex mode; a full-duplex port is considered to have a point-to-point connection and a half-duplex port is considered to have a shared connection. You can override the default setting that is controlled by the duplex setting by entering the spanning-tree link-type interface configuration command.
This proposal/agreement handshake is initiated only when a nonedge port moves from the blocking to the forwarding state. The handshaking process then proliferates step-by-step throughout the topology.
Protocol Timers
Table 4-2 describes the protocol timers that affect the Rapid PVST+ performance.
Port Roles
Rapid PVST+ provides rapid convergence of the spanning tree by assigning port roles and learning the active topology. Rapid PVST+ builds upon the 802.1D STP to select the device with the highest switch priority (lowest numerical priority value) as the root bridge as described in the "Election of the Root Bridge" section. Rapid PVST+ then assigns one of these port roles to individual ports:
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In a stable topology with consistent port roles throughout the network, Rapid PVST+ ensures that every root port and designated port immediately transition to the forwarding state while all alternate and backup ports are always in the blocking state. Designated ports start in the blocking state. The port state controls the operation of the forwarding and learning processes.
A port with the root or a designated port role is included in the active topology. A port with the alternate or backup port role is excluded from the active topology (see Figure 4-5).
Figure 4-5 Sample Topology Demonstrating Port Roles
Port States
This section describes the Rapid PVST+ and MST port states and includes the following topics:
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Rapid PVST+ Port State Overview
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 a Layer 2 LAN port transitions directly from nonparticipation in the spanning tree topology to the forwarding state, it can create temporary data loops. Ports must wait for new topology information to propagate through the switched LAN before starting to forward frames.
Each Layer 2 LAN port on an NX-OS that uses Rapid PVST+ or MST, exists in one of the following four states:
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When you enable Rapid PVST+, every port in the device, VLAN, and network goes through the blocking state and the transitory states of learning at power up. If properly configured, each Layer 2 LAN port stabilizes to the forwarding or blocking state.
When the STP algorithm places a Layer 2 LAN port in the forwarding state, the following process occurs:
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Blocking State
A Layer 2 LAN port in the blocking state does not participate in frame forwarding.
A Layer 2 LAN port in the blocking state performs as follows:
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Learning State
A Layer 2 LAN port in the learning state prepares to participate in frame forwarding by learning the MAC addresses for the frames. The Layer 2 LAN port enters the learning state from the blocking state.
A Layer 2 LAN port in the learning state performs as follows:
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Forwarding State
A Layer 2 LAN port in the forwarding state forwards frames. The Layer 2 LAN port enters the forwarding state from the learning state.
A Layer 2 LAN port in the forwarding state performs as follows:
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Disabled State
A Layer 2 LAN port in the disabled state does not participate in frame forwarding or STP. A Layer 2 LAN port in the disabled state is virtually nonoperational.
A disabled Layer 2 LAN port performs as follows:
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Summary of Port States
Table 4-3 lists the possible operational and Rapid PVST+ states for ports and whether the port is included in the active topology.
Table 4-3 Port State Active Topology
Active Topology?Enabled
Blocking
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Forwarding
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Synchronization of Port Roles
When the device receives a proposal message on one of its ports and that port is selected as the new root port, Rapid PVST+ forces all other ports to synchronize with the new root information.
The device is synchronized with superior root information received on the root port if all other ports are synchronized. An individual port on the device is synchronized if either of the following applies:
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If a designated port is in the forwarding state and is not configured as an edge port, it transitions to the blocking state when the Rapid PVST+ forces it to synchronize with new root information. In general, when the Rapid PVST+ forces a port to synchronize with root information and the port does not satisfy any of the above conditions, its port state is set to blocking.
After ensuring that all of the ports are synchronized, the device sends an agreement message to the designated device that corresponds to its root port. When the switches connected by a point-to-point link are in agreement about their port roles, Rapid PVST+ immediately transitions the port states to the forwarding state. The sequence of events is shown in Figure 4-6.
Figure 4-6 Sequence of Events During Rapid Convergence
This section includes the following topics:
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Processing Superior BPDU Information
A superior BPDU is a BPDU with root information (such as a lower switch ID or lower path cost) that is superior to what is currently stored for the port.
If a port receives a superior BPDU, Rapid PVST+ triggers a reconfiguration. If the port is proposed and is selected as the new root port, Rapid PVST+ forces all the designated, nonedge ports to synchronize.
If the received BPDU is a Rapid PVST+ BPDU with the proposal flag set, the device sends an agreement message after all of the other ports are synchronized. The new root port transitions to the forwarding state as soon as the previous port reaches the blocking state.
If the superior information received on the port causes the port to become a backup port or an alternate port, Rapid PVST+ sets the port to the blocking state and sends an agreement message. The designated port continues sending BPDUs with the proposal flag set until the forward-delay timer expires. At that time, the port transitions to the forwarding state.
Processing Inferior BPDU Information
An inferior BPDU is a BPDU with root information (such as a higher switch ID or higher path cost) that is inferior to what is currently stored for the port.
If a designated port receives an inferior BPDU, it immediately replies with its own information.
Detecting Unidirectional Link Failure
The software checks the consistency of the port role and state in the received BPDUs to detect unidirectional link failures that could cause bridging loops using Unidirectional Link Detection (UDLD) feature. This feature is based on the dispute mechanism.
See the Cisco DCNM Interface Configuration Guide for information on UDLD.
When a designated port detects a conflict, it keeps its role, but reverts to a discarding state because disrupting connectivity in case of inconsistency is preferable to opening a bridging loop.
Figure 4-7 illustrates a unidirectional link failure that typically creates a bridging loop. Switch A is the root bridge, and its BPDUs are lost on the link leading to device B. The 802.1w-standard BPDUs include the role and state of the sending port. With this information, switch A can detect that switch B does not react to the superior BPDUs that it sends and that switch B is the designated, not root port. As a result, switch A blocks (or keeps blocking) its port, which prevents the bridging loop.
Figure 4-7 Detecting Unidirectional Link Failure
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Port Cost
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The STP port path-cost default value is determined from the media speed and path-cost calculation method of a LAN interface (see Table 4-4). If a loop occurs, STP considers the port cost when selecting a LAN interface to put into the forwarding state.
You can assign the lower cost values to LAN interfaces that you want STP to select first and higher cost values to LAN interfaces that you want STP to select last. If all LAN interfaces have the same cost value, STP puts the LAN interface with the lowest LAN interface number in the forwarding state and blocks other LAN interfaces.
On access ports, you assign port cost by the port. On trunk ports, you assign the port cost by the VLAN; you can configure the same port cost to all the VLANs on a trunk port.
Port Priority
If a redundant path occurs and multiple ports have the same path cost, Rapid PVST+ considers the port priority when selecting which LAN port to put into the forwarding state. You can assign lower priority values to LAN ports that you want Rapid PVST+ to select first and higher priority values to LAN ports that you want Rapid PVST+ to select last.
If all LAN ports have the same priority value, Rapid PVST+ puts the LAN port with the lowest LAN port number in the forwarding state and blocks other LAN ports. The possible priority range is from 0 through 224 (the default is128), configurable in increments of 32. The NX-OS device uses the port priority value when the LAN port is configured as an access port and uses VLAN port priority values when the LAN port is configured as a trunk port.
Rapid PVST+ and IEEE 802.1Q Trunks
The 802.1Q trunks impose some limitations on the STP strategy for a network. In a network of Cisco network devices connected through 802.1Q trunks, the network devices maintain one instance of STP for each VLAN allowed on the trunks. However, non-Cisco 802.1Q network devices maintain only one instance of STP for all VLANs allowed on the trunks, which is the Common Spanning Tree (CST).
When you connect a Cisco network device to a non-Cisco device through an 802.1Q trunk, the Cisco network device combines the STP instance of the 802.1Q VLAN of the trunk with the STP instance of the non-Cisco 802.1Q network device. However, all per-VLAN STP information that is maintained by Cisco network devices is separated by a cloud of non-Cisco 802.1Q network devices. The non-Cisco 802.1Q cloud that separates the Cisco network devices is treated as a single trunk link between the network devices.
For more information on 802.1Q trunks, see the Cisco NX-OS Interface Configuration Guide.
Rapid PVST+ Interoperation with Legacy 802.1D STP
Rapid PVST+ can interoperate with devices that are running the legacy 802.1D protocol. The device knows that it is interoperating with equipment running 802.1D when it receives a BPDU version 0. The BPDUs for Rapid PVST+ are version 2. If the BPDU received is an 802.1w BPDU version 2 with the proposal flag set, the device sends an agreement message after all of the other ports are synchronized. If the BPDU is an 802.1D BPDU version 0, the device does not set the proposal flag and starts the forward-delay timer for the port. The new root port requires twice the forward-delay time to transition to the forwarding state.
The device interoperates with legacy 802.1D devices as follows:
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This method of operation is required only for 802.1D switches. The 802.1w BPDUs do not have the TCA bit set.
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When a port is initialized, the migrate-delay timer is started (specifies the minimum time during which 802.1w BPDUs are sent), and 802.1w BPDUs are sent. While this timer is active, the device processes all BPDUs received on that port and ignores the protocol type.
If the device receives an 802.1D BPDU after the port migration-delay timer has expired, it assumes that it is connected to an 802.1D device and starts using only 802.1D BPDUs. However, if the 802.1w device is using 802.1D BPDUs on a port and receives an 802.1w BPDU after the timer has expired, it restarts the timer and starts using 802.1w BPDUs on that port.
Rapid PVST+ Interoperation with 802.1s MST
Rapid PVST+ interoperates seamlessly with the IEEE 802.1s Multiple Spanning Tree (MST) standard. No user configuration is needed. To disable this seamless interoperation, see Chapter 6, "Configuring STP Extensions" for information on PVST Simulation.
High Availability
The software supports high availability for Rapid PVST+. However, the statistics and timers are not restored when Rapid PVST+ restarts. The timers start again and the statistics begin from 0.
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Virtualization Support
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The system provides support for virtual device contexts (VDCs). Using VDCs, you have a separate Layer 2 virtualization in each VDC, and each VDC runs a separate STP (See Figure 4-8).
Figure 4-8 Separate STP in each VDC
Each VDC will have its own Rapid PVST+. You cannot configure Rapid PVST+ across VDCs with NX-OS software. However, you can ruRapid PVST+ in one VDC and run MST in each VDC. Ensure that you are in the correct VDC before you begin configuring either Rapid PVST+ or MST parameters.
For example, VDC1 can run MST, VDC2 can run Rapid PVST+, and VDC3 can run MST.
Licensing Requirements for Rapid PVST+
The following table shows the licensing requirements for this feature:
However, using VDCs requires an Advanced Services license.
Prerequisites for Configuring Rapid PVST+
Rapid PVST+ has the following prerequisites:
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Guidelines and Limitations
Follow these limitations and restrictions when configuring Rapid PVST:
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Configuring Rapid PVST+
Rapid PVST+, which has the 802.1 w standard applied to the PVST+ protocol, is the default STP setting in the device.
You enable Rapid PVST+ on a per-VLAN basis. The device maintains a separate instance of STP for each VLAN (except on those VLANS on which you disable STP). By default, Rapid PVST+ is enabled on the default VLAN and on each VLAN that you create.
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This section includes the following topics:
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Enabling Rapid PVST+
If you disable Rapid PVST+ on any VLANs, you must reenable Rapid PVRST+ on the specified VLANs. If you have enabled MST on the device and now want to use Rapid PVST+, you must enable Rapid PVST+ on the device.
(See the "Disabling or Enabling Rapid PVST+ per VLAN" section.)
Rapid PVST+ is the default STP mode. You cannot simultaneously run MST and Rapid PVST+ in the same VDC.
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You enable Rapid PVST+ using the Spanning Tree pane (see Figure 4-9).
Figure 4-9 Enabling Rapid PVST+
DETAILED STEPS
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To enable Rapid PVST+ on the device, follow these steps:
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Rapid PVST+ is enabled on all VLANs by default.
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Setting All STP Parameters on the Device to Default Values
You can set all the STP parameters on a device to default values, which set the STP mode to Rapid PVST+.
You use the Spanning Tree pane to set all STP parameters on the device to default values (see Figure 4-9).
DETAILED STEPS
To set all the STP parameters on the device to the default values, follow these steps:
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Rapid PVST+ is the default STP mode.
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Disabling or Enabling Rapid PVST+ per VLAN
You can enable or disable Rapid PVST+ on each VLAN.
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You use the Rapid-PVST+ pane to enable or disable Rapid PVST+ per VLAN (see Figure 4-10).
Figure 4-10 Configuring Rapid PVST+
DETAILED STEPS
To enable or disable Rapid PVST+ per VLAN, follow these steps:
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Rapid PVST+ is enabled on all VLANs by default.
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Caution
Configuring the Primary and Secondary Roots and the Switch Priority
The software maintains a separate instance of STP for each active VLAN in Rapid PVST+. For each VLAN, the network device with the lowest bridge ID becomes the root bridge for that VLAN.
You can configure the primary and secondary roots, or you can configure the switch priority for the specified VLAN so that it is more likely that the specified device is chosen as the root bridge.
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To configure multiple backup root bridges, designate more than one device as secondary roots. For the secondary root, use the same network diameter and hello-time values that you used when you configured the primary root bridge.
You use the Rapid-PVST+ pane to set the device to become the primary root bridge for the VLAN (see Figure 4-10).
DETAILED STEPS
To configure a device to become the primary or secondary root bridge for a VLAN in Rapid PVST+ or to configure the switch priority, follow these steps:
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Tabs appear in the Details pane.
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The VLAN Setting section has a Switch Priority area that allows you to set the device as the primary or secondary root for the specified VLAN, as well as setting the switch priority.
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The default value is 32768.
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The default value for the diameter is 7.
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The default for the hello time is 2 seconds.
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Configuring the Rapid PVST+ Port Priority
You can assign lower priority values to LAN ports that you want Rapid PVST+ to select first and higher priority values to LAN ports that you want Rapid PVST+ to select last. If all LAN ports have the same priority value, Rapid PVST+ puts the LAN port with the lowest LAN port number in the forwarding state and blocks other LAN ports.
The device uses the port priority value when the LAN port is configured as an access port and uses the VLAN port priority values when the LAN port is configured as a trunk port.
You use the Rapid-PVST+ pane to assign port priority values in Rapid PVST+ (see Figure 4-10).
DETAILED STEPS
To assign Rapid PVST+ port priorities to individual ports, follow these steps:
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Tabs appear in the Details pane.
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The default value is 128.
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Configuring the Rapid PVST+ Path-Cost Method and Port Cost
On access ports, you can assign the port cost for each port. On trunk ports, you can assign the port cost for each VLAN; you can configure all the VLANs on a trunk with the same port cost.
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You use the Spanning Tree pane to configure the path-cost method for Rapid PVST+ (see Figure 4-9).
You use the Rapid-PVST+ pane to set the Rapid PVST+ cost for a port (see Figure 4-10).
DETAILED STEPS
To set the path-cost method and port cost in Rapid PVST+, follow these steps:
Step 1
Step 2
Tabs appear in the Details pane.
Step 3
Step 4
Step 5
The default path-cost method is short.
Step 6
Step 7
Step 8
Step 9
Tabs appear in the Details pane.
Step 10
Step 11
Step 12
Step 13
The default value is 128.
Step 14
Valid values for the short path-cost method, which is the default method, are from 1 to 65535. Valid values for the long path-cost method are from 1 to 200000000. The default port cost is determined by the port's bandwidth.
Step 15
Setting All Rapid PVST+ Parameters to Default Values per Interface
You can use the Rapid-PVST+ pane to reset all STP parameters on the selected interface for the selected VLAN on the device to default Rapid PVST+ values (see Figure 4-11).
Figure 4-11 Setting Rapid PVST+ Values
DETAILED STEPS
To set all the STP parameters on the device for an interface for the specified VLAN to default values, follow these steps:
Step 1
Step 2
Step 3
The devices that carry that VLAN appear.
Step 4
The system highlights the device in the Summary pane, and tabs appear in the Details pane.
Step 5
Step 6
The interfaces in the VLAN on the selected device appear.
Step 7
The system highlights the selected interface.
Step 8
Step 9
Configuring the Rapid PVST+ Hello Time for a VLAN
You can configure the Rapid PVST+ hello time for a VLAN.
Note
You use the Rapid-PVST+ pane to configure the hello time for the VLAN (see Figure 4-10).
DETAILED STEPS
To configure the hello time for a VLAN in Rapid PVST+, follow these steps:
Step 1
Step 2
Step 3
Tabs appear in the Details pane.
Step 4
Step 5
Step 6
The default for the hello time is 2 seconds.
Step 7
Configuring the Rapid PVST+ Forward Delay Time for a VLAN
You can configure the forward delay time per VLAN when using Rapid PVST+.
You use the Rapid-PVST+ pane to configure the forward delay time for the VLAN (see Figure 4-10).
DETAILED STEPS
To configure the forward delay time for a VLAN in Rapid PVST+, follow these steps:
Step 1
Step 2
Step 3
Tabs appear in the Details pane.
Step 4
Step 5
Step 6
The default for the forward delay time is 15 seconds.
Step 7
Configuring the Rapid PVST+ Maximum Age Time for a VLAN
You can configure the maximum age time per VLAN when using Rapid PVST+.
You use the Rapid-PVST+ pane to configure the maximum age time for the VLAN (see Figure 4-10).
DETAILED STEPS
To configure the maximum age time for a VLAN in Rapid PVST+, follow these steps:
Step 1
Step 2
Step 3
Tabs appear in the Details pane.
Step 4
Step 5
Step 6
The default for the maximum age time is 20 seconds.
Step 7
Specifying the Link Type
Rapid connectivity (802.1w standard) is established only on point-to-point links. By default, the link type is controlled from the duplex mode of the interface. A full-duplex port is considered to have a point-to-point connection; a half-duplex port is considered to have a shared connection.
If you have a half-duplex link physically connected point to point to a single port on a remote device, you can override the default setting on the link type and enable rapid transitions.
If you set the link to shared, STP falls back to 802.1D.
You use the Spanning Tree pane to set the link type (see Figure 4-9).
DETAILED STEPS
To configure the link type, follow these steps:
Step 1
Step 2
Step 3
Step 4
The Port Setting section expands.
Step 5
Step 6
The default Link Type is Auto.
Step 7
Displaying Statistics
The following window appears in the Statistics tab:
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Field Descriptions for Rapid PVST+
These field descriptions are used for configuring Rapid PVST+. This section includes the following topics:
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Device View: VLAN: Details: VLAN Setting Section
Device View: VLAN: Details: Port Setting Section
Network View: Device: Details: VLAN Setting Section
Network View: Device: Details: Port Setting Section
Additional References
For additional information related to implementing Rapid PVST+, see the following sections:
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Related Documents
Standards
IEEE 802.1Q-2006 (formerly known as IEEE 802.1s), IEEE 802.1D-2004 (formerly known as IEEE 802.1w), IEEE 802.1D, IEEE 802.1t
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MIBs
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To locate and download MIBs, go to the following URL:
http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml
