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Table Of Contents
Understanding Bridge Protocol Data Units
Creating the Spanning Tree Topology
Understanding IEEE 802.1w RSTP
Port Roles and the Active Topology
Bridge Protocol Data Unit Format and Processing
BPDU Format and Processing Overview
Processing Superior BPDU Information
Processing Inferior BPDU Information
Spanning Tree Operation Within an MST Region
Spanning Tree Operations Between MST Regions
Standard-Compliant MST Implementation
Spanning Tree Interoperation Between Legacy and Standard-Compliant Switches
Detecting Unidirectional Link Failure
Interoperability with IEEE 802.1D-1998 STP
Enabling the Extended System ID
Configuring a Secondary Root Bridge
Configuring the Bridge Priority of a VLAN
Configuring the Forward-Delay Time for a VLAN
Configuring the Maximum Aging Time for a VLAN
MST Configuration Guidelines and Restrictions
Specifying the MST Region Configuration and Enabling MST
Configuring a Secondary Root Bridge
Configuring the Switch Priority
Configuring the Forwarding-Delay Time
Configuring the Transmit Hold Count
Configuring the Maximum-Aging Time
Configuring the Maximum-Hop Count
Specifying the Link Type to Ensure Rapid Transitions
Restarting the Protocol Migration Process
Displaying the MST Configuration and Status
Configuring STP and MST
This chapter describes how to configure the Spanning Tree Protocol (STP) and Multiple Spanning Tree (MST) protocol in Cisco IOS Software Release 12.2SX.
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For complete syntax and usage information for the commands used in this chapter, see the Cisco IOS Software Releases 12.2SX Command References at this URL:
http://www.cisco.com/en/US/docs/ios/mcl/122sx_mcl.html
This chapter consists of these sections:
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Understanding STP
These sections describe how STP works:
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STP Overview
STP is a Layer 2 link-management protocol that provides path redundancy while preventing undesirable loops in the network. 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.
Layer 2 Ethernet ports use STP (the IEEE 802.1D bridge protocol) on all VLANs. By default, a single instance of STP runs on each configured VLAN (provided you do not manually disable STP). You can enable and disable STP on a per-VLAN basis.
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 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. These conditions result in an unstable network.
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 standby (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 standby 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 is put in the forwarding state and which port is put in the blocking state. The STP port priority value represents the location of a port in the network topology and how efficiently that location allows the port to pass traffic. The STP port path cost value represents media speed.
Understanding the Bridge ID
Each VLAN on each network device has a unique 64-bit bridge ID consisting of a bridge priority value, an extended system ID, and an STP MAC address allocation.
This section contains these topics:
Bridge Priority Value
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The bridge priority is a 4-bit value when the extended system ID is enabled (see Table 22-1 and the "Configuring the Bridge Priority of a VLAN" section).
Extended System ID
A 12-bit extended system ID field is part of the bridge ID (see Table 22-1). Chassis that support only 64 MAC addresses always use the 12-bit extended system ID. On chassis that support 1024 MAC addresses, you can enable use of the extended system ID. STP uses the VLAN ID as the extended system ID. See the "Enabling the Extended System ID" section.
STP MAC Address Allocation
Catalyst 6500 series switch chassis have either 64 or 1024 MAC addresses available to support software features such as STP. To view the MAC address range on your chassis, enter the show catalyst6000 chassis-mac-address command.
For chassis with 64 MAC addresses, STP uses the extended system ID plus a MAC address to make the bridge ID unique for each VLAN.
When the extended system ID is not enabled, STP uses one MAC address per VLAN to make the bridge ID unique for each VLAN.
If you have a network device in your network with the extended system ID enabled, you should also enable the extended system ID on all other Layer 2 connected network devices to avoid undesirable root bridge election and spanning tree topology issues.
When the extended system ID is enabled, the root bridge priority becomes a multiple of 4096 plus the VLAN ID. With the extended system ID enabled, a switch bridge ID (used by the spanning tree algorithm to determine the identity of the root bridge, the lowest being preferred) can only be specified as a multiple of 4096. Only the following values are possible: 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440.
If another bridge in the same spanning tree domain does not have the extended system ID enabled, it could win root bridge ownership because of the finer granularity in the selection of its bridge ID.
Understanding Bridge Protocol Data Units
Bridge protocol data units (BPDUs) are transmitted in one direction from the root bridge. 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 BPDU frame, all network devices connected to the LAN 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, and, if the topology changes, initiate a BPDU transmission.
A BPDU exchange results in the following:
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Election of the Root Bridge
For each VLAN, the network device with the highest-priority bridge ID (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 switch will be elected as the root bridge. Configuring a higher-priority value increases the probability; a lower-priority value decreases the probability.
The STP root bridge is the logical center of the 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 Layer 2 network, to elect the root port leading to the root bridge, and to determine the designated port for each Layer 2 segment.
STP Protocol Timers
Table 22-2 describes the STP protocol timers that affect STP performance.
Creating the Spanning Tree Topology
In Figure 22-1, 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, 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 22-1 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 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.
STP Port States
These sections describe the STP port states:
STP 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. They must allow the frame lifetime to expire for frames that have been forwarded using the old topology.
Each Layer 2 LAN port using STP exists in one of the following five states:
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A Layer 2 LAN port moves through these five states as follows:
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Figure 22-2 illustrates how a Layer 2 LAN port moves through the five states.
Figure 22-2 STP Layer 2 LAN Interface States
When you enable STP, every port, VLAN, and network goes through the blocking state and the transitory states of listening and 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, as shown in Figure 22-3. After initialization, a BPDU is sent out to each Layer 2 LAN port. A network device initially assumes it is the root until it exchanges BPDUs with other network devices. This exchange establishes which network device in the network is the root or root bridge. If only one network device is in the network, no exchange occurs, the forward delay timer expires, and the ports move to the listening state. A port always enters the blocking state following initialization.
Figure 22-3 Interface 2 in Blocking State
A Layer 2 LAN port in the blocking state performs as follows:
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Listening State
The listening state is the first transitional state a Layer 2 LAN port enters after the blocking state. The Layer 2 LAN port enters this state when STP determines that the Layer 2 LAN port should participate in frame forwarding. Figure 22-4 shows a Layer 2 LAN port in the listening state.
Figure 22-4 Interface 2 in Listening State
A Layer 2 LAN port in the listening 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. The Layer 2 LAN port enters the learning state from the listening state. Figure 22-5 shows a Layer 2 LAN port in the learning state.
Figure 22-5 Interface 2 in Learning 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, as shown in Figure 22-6. The Layer 2 LAN port enters the forwarding state from the learning state.
Figure 22-6 Interface 2 in Forwarding 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, as shown in Figure 22-7. A Layer 2 LAN port in the disabled state is virtually nonoperational.
Figure 22-7 Interface 2 in Disabled State
A disabled Layer 2 LAN port performs as follows:
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STP and IEEE 802.1Q Trunks
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.
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 is maintained by Cisco network devices separated by a cloud of non-Cisco 802.1Q network devices. The non-Cisco 802.1Q cloud separating the Cisco network devices is treated as a single trunk link between the network devices.
For more information on 802.1Q trunks, see Chapter 13, "Configuring LAN Ports for Layer 2 Switching."
Understanding IEEE 802.1w RSTP
RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree. Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the default settings in the 802.1D spanning tree).
These section describes how the RSTP works:
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Port Roles and the Active Topology
The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learning the active topology. The RSTP builds upon the 802.1D STP to select the switch with the highest switch priority (lowest numerical priority value) as the root bridge as described in the "Election of the Root Bridge" section. The RSTP then assigns one of these port roles to individual ports:
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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.
In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root port and designated port immediately transition to the forwarding state while all alternate and backup ports are always in the discarding state (equivalent to blocking in 802.1D). The port state controls the operation of the forwarding and learning processes. Table 22-3 provides a comparison of 802.1D and RSTP port states.
To be consistent with Cisco STP implementations, this guide defines the port state as blocking instead of discarding. Designated ports start in the listening state.
Rapid Convergence
The RSTP provides for rapid recovery of connectivity following the failure of a switch, a switch 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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As shown in Figure 22-8, 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. Switch A sends a proposal message (a configuration BPDU with the proposal flag set) to switch B, proposing itself as the designated switch.
After receiving the proposal message, switch 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 switch B's agreement message, switch A also immediately transitions its designated port to the forwarding state. No loops in the network are formed because switch B blocked all of its nonedge ports and because there is a point-to-point link between switches A and B.
When switch C is connected to switch B, a similar set of handshaking messages are exchanged. Switch C selects the port connected to switch B as its root port, and both ends immediately transition to the forwarding state. With each iteration of this handshaking process, one more switch 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 using the spanning-tree link-type interface configuration command.
Figure 22-8 Proposal and Agreement Handshaking for Rapid Convergence
Synchronization of Port Roles
When the switch receives a proposal message on one of its ports and that port is selected as the new root port, the RSTP forces all other ports to synchronize with the new root information.
The switch is synchronized with superior root information received on the root port if all other ports are synchronized. An individual port on the switch is synchronized if:
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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 RSTP forces it to synchronize with new root information. In general, when the RSTP 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 switch sends an agreement message to the designated switch corresponding to its root port. When the switches connected by a point-to-point link are in agreement about their port roles, the RSTP immediately transitions the port states to forwarding. The sequence of events is shown in Figure 22-9.
Figure 22-9 Sequence of Events During Rapid Convergence
Bridge Protocol Data Unit Format and Processing
These sections describe bridge protocol data unit (BPDU) format and processing:
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BPDU Format and Processing Overview
The RSTP BPDU format is the same as the 802.1D BPDU format except that the protocol version is set to 2. A new 1-byte Version 1 Length field is set to zero, which means that no Version 1 protocol information is present. Table 22-4 describes the RSTP flag fields.
The sending switch sets the proposal flag in the RSTP BPDU to propose itself as the designated switch on that LAN. The port role in the proposal message is always set to the designated port.
The sending switch sets the agreement flag in the RSTP BPDU to accept the previous proposal. The port role in the agreement message is always set to the root port.
The RSTP does not have a separate TCN BPDU. It uses the topology change (TC) flag to show the topology changes. However, for interoperability with 802.1D switches, the RSTP switch processes and generates TCN BPDUs.
The learning and forwarding flags are set according to the state of the sending port.
Processing Superior BPDU Information
A superior BPDU is a BPDU with root information (such as 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, the RSTP triggers a reconfiguration. If the port is proposed and is selected as the new root port, RSTP forces all the other ports to synchronize.
If the BPDU received is an RSTP BPDU with the proposal flag set, the switch sends an agreement message after all of the other ports are synchronized. If the BPDU is an 802.1D BPDU, the switch 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.
If the superior information received on the port causes the port to become a backup port or an alternate port, RSTP 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 which time the port transitions to the forwarding state.
Processing Inferior BPDU Information
An inferior BPDU is a BPDU with root information (such as 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.
Topology Changes
These are the differences between the RSTP and the 802.1D in handling spanning tree topology changes:
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This method of operation is only required to support 802.1D switches. The RSTP BPDUs never 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 RSTP BPDUs are sent), and RSTP BPDUs are sent. While this timer is active, the switch processes all BPDUs received on that port and ignores the protocol type.
If the switch receives an 802.1D BPDU after the port migration-delay timer has expired, it assumes that it is connected to an 802.1D switch and starts using only 802.1D BPDUs. However, if the RSTP switch is using 802.1D BPDUs on a port and receives an RSTP BPDU after the timer has expired, it restarts the timer and starts using RSTP BPDUs on that port.
Rapid-PVST
Rapid-PVST uses the existing configuration for PVST+; however, Rapid-PVST uses RSTP to provide faster convergence. Independent VLANs run their own RSTP instance.
Dynamic entries are flushed immediately on a per-port basis upon receiving a topology change.
UplinkFast and BackboneFast configurations are ignored in Rapid-PVST mode; both features are included in RSTP.
Understanding MST
These sections describe MST:
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MST Overview
MST maps multiple VLANs into a spanning tree instance, with each instance having a spanning tree topology independent of other spanning tree instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing, and reduces the number of spanning tree instances required to support a large number of VLANs. MST improves the fault tolerance of the network because a failure in one instance (forwarding path) does not affect other instances (forwarding paths).
The most common initial deployment of MST is in the backbone and distribution layers of a Layer 2 switched network. This deployment provides the kind of highly available network that is required in a service-provider environment.
MST provides rapid spanning tree convergence through explicit handshaking, which eliminates the 802.1D forwarding delay and quickly transitions root bridge ports and designated ports to the forwarding state.
MST improves spanning tree operation and maintains backward compatibility with these STP versions:
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For information about PVST+ and rapid PVST+, see "Configuring STP and MST." For information about other spanning tree features such as Port Fast, UplinkFast, root guard, and so forth, see Chapter 23, "Configuring Optional STP Features."
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MST Regions
For switches to participate in MST instances, you must consistently configure the switches with the same MST configuration information. A collection of interconnected switches that have the same MST configuration comprises an MST region as shown in Figure 22-10.
The MST configuration controls to which MST region each switch belongs. The configuration includes the name of the region, the revision number, and the MST VLAN-to-instance assignment map.
A region can have one or multiple members with the same MST configuration; each member must be capable of processing RSTP bridge protocol data units (BPDUs). There is no limit to the number of MST regions in a network, but each region can support up to 65 spanning tree instances. Instances can be identified by any number in the range from 0 to 4094. You can assign a VLAN to only one spanning tree instance at a time.
IST, CIST, and CST
These sections describe internal spanning tree (IST), common and internal spanning tree (CIST), and common spanning tree (CST):
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IST, CIST, and CST Overview
Unlike other spanning tree protocols, in which all the spanning tree instances are independent, MST establishes and maintains IST, CIST, and CST spanning trees:
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Within each MST region, MST maintains multiple spanning tree instances. Instance 0 is a special instance for a region, known as the IST. All other MST instances are numbered from 1 to 4094.
The IST is the only spanning tree instance that sends and receives BPDUs. All of the other spanning tree instance information is contained in MSTP records (M-records), which are encapsulated within MST BPDUs. Because the MST BPDU carries information for all instances, the number of BPDUs that need to be processed to support multiple spanning tree instances is significantly reduced.
All MST instances within the same region share the same protocol timers, but each MST instance has its own topology parameters, such as root bridge ID, root path cost, and so forth. By default, all VLANs are assigned to the IST.
An MST instance is local to the region; for example, MST instance 1 in region A is independent of MST instance 1 in region B, even if regions A and B are interconnected.
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The spanning tree computed in a region appears as a subtree in the CST that encompasses the entire switched domain. The CIST is formed by the spanning tree algorithm running among switches that support the 802.1w, 802.1s, and 802.1D standards. The CIST inside an MST region is the same as the CST outside a region.
For more information, see the "Spanning Tree Operation Within an MST Region" section and the "Spanning Tree Operations Between MST Regions" section.
Spanning Tree Operation Within an MST Region
The IST connects all the MST switches in a region. When the IST converges, the root of the IST becomes the CIST regional root (called the IST master before the implementation of the 802.1s standard) as shown in Figure 22-10. The CIST regional root is also the CIST root if there is only one region in the network. If the CIST root is outside the region, one of the MST switches at the boundary of the region is selected as the CIST regional root.
When an MST switch initializes, it sends BPDUs that identify itself as the root of the CIST and the CIST regional root, with both of the path costs to the CIST root and to the CIST regional root set to zero. The switch also initializes all of its MST instances and claims to be the root for all of them. If the switch receives superior MST root information (lower switch ID, lower path cost, and so forth) than currently stored for the port, it relinquishes its claim as the CIST regional root.
During initialization, a region might have many subregions, each with its own CIST regional root. As switches receive superior IST information from a neighbor in the same region, they leave their old subregions and join the new subregion that contains the true CIST regional root, which causes all subregions to shrink except for the one that contains the true CIST regional root.
For correct operation, all switches in the MST region must agree on the same CIST regional root. Therefore, any two switches in the region only synchronize their port roles for an MST instance if they converge to a common CIST regional root.
Spanning Tree Operations Between MST Regions
If there are multiple regions or 802.1D switches within the network, MST establishes and maintains the CST, which includes all MST regions and all 802.1D STP switches in the network. The MST instances combine with the IST at the boundary of the region to become the CST.
The IST connects all the MST switches in the region and appears as a subtree in the CIST that encompasses the entire switched domain. The root of the subtree is the CIST regional root. The MST region appears as a virtual switch to adjacent STP switches and MST regions.
Figure 22-10 shows a network with three MST regions and an 802.1D switch (D). The CIST regional root for region 1 (A) is also the CIST root. The CIST regional root for region 2 (B) and the CIST regional root for region 3 (C) are the roots for their respective subtrees within the CIST.
Figure 22-10 MST Regions, CIST Regional Roots, and CST Root
Only the CST instance sends and receives BPDUs, and MST instances add their spanning tree information into the BPDUs to interact with neighboring switches and compute the final spanning tree topology. Because of this, the spanning tree parameters related to BPDU transmission (for example, hello time, forward time, max-age, and max-hops) are configured only on the CST instance but affect all MST instances. Parameters related to the spanning tree topology (for example, switch priority, port VLAN cost, and port VLAN priority) can be configured on both the CST instance and the MST instance.
MST switches use Version 3 BPDUs or 802.1D STP BPDUs to communicate with 802.1D switches. MST switches use MST BPDUs to communicate with MST switches.
IEEE 802.1s Terminology
Some MST naming conventions used in the prestandard implementation have been changed to include identification of some internal and regional parameters. These parameters are used only within an MST region, compared to external parameters that are used throughout the whole network. Because the CIST is the only spanning tree instance that spans the whole network, only the CIST parameters require the external qualifiers and not the internal or regional qualifiers.
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Table 22-5 compares the IEEE standard and the Cisco prestandard terminology.
Hop Count
MST does not use the message-age and maximum-age information in the configuration BPDU to compute the spanning tree topology. Instead, they use the path cost to the root and a hop-count mechanism similar to the IP time-to-live (TTL) mechanism.
By using the spanning-tree mst max-hops global configuration command, you can configure the maximum hops inside the region and apply it to the IST and all MST instances in that region. The hop count achieves the same result as the message-age information (triggers a reconfiguration). The root bridge of the instance always sends a BPDU (or M-record) with a cost of 0 and the hop count set to the maximum value. When a switch receives this BPDU, it decrements the received remaining hop count by one and propagates this value as the remaining hop count in the BPDUs it generates. When the count reaches zero, the switch discards the BPDU and ages the information held for the port.
The message-age and maximum-age information in the RSTP portion of the BPDU remain the same throughout the region, and the same values are propagated by the region-designated ports at the boundary.
Boundary Ports
In the Cisco prestandard implementation, a boundary port connects an MST region to one of these STP regions:
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A boundary port also connects to a LAN, the designated switch of which is either a single spanning tree switch or a switch with a different MST configuration.
There is no definition of a boundary port in the 802.1s standard. The 802.1Q-2002 standard identifies two kinds of messages that a port can receive: internal (coming from the same region) and external. When a message is external, it is received only by the CIST. If the CIST role is root or alternate, or if the external BPDU is a topology change, it could have an impact on the MST instances. When a message is internal, the CIST part is received by the CIST, and each MST instance receives its respective M-record. The Cisco prestandard implementation treats a port that receives an external message as a boundary port, which means a port cannot receive a mix of internal and external messages.
An MST region includes both switches and LANs. A segment belongs to the region of its designated port. Therefore, a port in a different region from the designated port for a segment is a boundary port. This definition allows two ports internal to a region to share a segment with a port belonging to a different region, creating the possibility of receiving both internal and external messages on a port.
The primary change from the Cisco prestandard implementation is that a designated port is not defined as boundary unless it is running in an STP-compatible mode.
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The other change from the prestandard implementation is that the CIST regional root bridge ID field is now inserted where an RSTP or legacy 802.1s switch has the sender switch ID. The whole region performs like a single virtual switch by sending a consistent sender switch ID to neighboring switches. In this example, switch C would receive a BPDU with the same consistent sender switch ID of root, whether or not A or B is designated for the segment.
Standard-Compliant MST Implementation
The standard-compliant MST implementation includes features required to meet the standard, as well as some of the desirable prestandard functionality that is not yet incorporated into the published standard. These sections describe the standard-compliant MST implementation:
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Changes in Port-Role Naming
The boundary role was deleted from the final MST standard, but this boundary concept is maintained in the standard-compliant implementation. However, an MST instance (MSTI) port at a boundary of the region might not follow the state of the corresponding CIST port. The following two situations currently exist:
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Spanning Tree Interoperation Between Legacy and Standard-Compliant Switches
Because automatic detection of prestandard switches can fail, you can use an interface configuration command to identify prestandard ports. A region cannot be formed between a standard and a prestandard switch, but they can interoperate before using the CIST. Only the capability of load balancing over different instances is lost in this specific situation. The CLI displays different flags depending on the port configuration when the port receives prestandard BPDUs. A syslog message also appears the first time a switch receives a prestandard BPDU on a port that has not been configured for prestandard BPDU transmission.
Figure 22-11 illustrates a standard-compliant switch connected to a prestandard switch. Assume that A is the standard-compliant switch and B is a prestandard switch, both configured to be in the same region. A is the root bridge for the CIST, and so B has a root port (BX) on segment X and an alternate port (BY) on segment Y. If segment Y flaps, and the port on BY becomes the alternate before sending out a single prestandard BPDU, AY cannot detect that a prestandard switch is connected to Y and continues to send standard BPDUs. The port BY is fixed in a boundary, and no load balancing is possible between A and B. The same problem exists on segment X, but B might transmit topology changes.
Figure 22-11 Standard-Compliant and Prestandard Switch Interoperation
Note
Detecting Unidirectional Link Failure
This feature is not yet present in the IEEE MST standard, but it is included in the standard-compliant implemtation. 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.
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 22-12 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 switch B. RSTP and MST 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 it sends and that switch B is the designated, not root bridge. As a result, switch A blocks (or keeps blocking) its port, thus preventing the bridging loop.
Figure 22-12 Detecting Unidirectional Link Failure
Interoperability with IEEE 802.1D-1998 STP
A switch running MST supports a built-in protocol migration feature that enables it to interoperate with 802.1D switches. If this switch receives an 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MST switch also can detect that a port is at the boundary of a region when it receives an 802.1D BPDU, an MST BPDU (Version 3) associated with a different region, or an RSTP BPDU (Version 2).
However, the switch does not automatically revert to the MST mode if it no longer receives 802.1D BPDUs because it cannot detect whether the 802.1D switch has been removed from the link unless the 802.1D switch is the designated switch. A switch might also continue to assign a boundary role to a port when the switch to which this switch is connected has joined the region. To restart the protocol migration process (force the renegotiation with neighboring switches), use the clear spanning-tree detected-protocols privileged EXEC command.
If all the 802.1D switches on the link are RSTP switches, they can process MST BPDUs as if they are RSTP BPDUs. Therefore, MST switches send either a Version 0 configuration and topology change notification (TCN) BPDUs or Version 3 MST BPDUs on a boundary port. A boundary port connects to a LAN, the designated switch of which is either a single spanning tree switch or a switch with a different MST configuration.
Configuring STP
These sections describe how to configure STP on VLANs:
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Caution
Default STP Configuration
Table 22-6 shows the default STP configuration.
Enabling STP
Note
You can enable STP on a per-VLAN basis. The switch maintains a separate instance of STP for each VLAN (except on VLANs on which you disable STP).
To enable STP on a per-VLAN basis, perform this task:
Step 1
Router(config)# spanning-tree vlan vlan_ID
Enables STP on a per-VLAN basis. The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 22-6).
Router(config)# default spanning-tree vlan vlan_ID
Reverts all STP parameters to default values for the specified VLAN.
Router(config)# no spanning-tree vlan vlan_ID
Disables STP on the specified VLAN; see the following Cautions for information regarding this command.
Step 2
Router(config)# end
Exits configuration mode.
Step 3
Router# show spanning-tree vlan vlan_ID
Verifies that STP is enabled.
Caution
Caution
This example shows how to enable STP on VLAN 200:
Router# configure terminalRouter(config)# spanning-tree vlan 200Router(config)# endRouter#
Note
This example shows how to verify the configuration:
Router# show spanning-tree vlan 200VLAN0200Spanning tree enabled protocol ieeeRoot ID Priority 32768Address 00d0.00b8.14c8This bridge is the rootHello Time 2 sec Max Age 20 sec Forward Delay 15 secBridge ID Priority 32768Address 00d0.00b8.14c8Hello Time 2 sec Max Age 20 sec Forward Delay 15 secAging Time 300Interface Role Sts Cost Prio.Nbr Status---------------- ---- --- --------- -------- --------------------------------Gi1/4 Desg FWD 200000 128.196 P2pGi1/5 Back BLK 200000 128.197 P2pRouter#
Note
Enabling the Extended System ID
Note
You can enable the extended system ID on chassis that support 1024 MAC addresses (see the "Understanding the Bridge ID" section).
To enable the extended system ID, perform this task:
Step 1
Router(config)# spanning-tree extend system-id
Enables the extended system ID.
Router(config)# no spanning-tree extend system-id
Disables the extended system ID.
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Step 2
Router(config)# end
Exits configuration mode.
Step 3
Router# show spanning-tree vlan vlan_ID
Verifies the configuration.
Note
This example shows how to enable the extended system ID:
Router# configure terminalRouter(config)# spanning-tree extend system-idRouter(config)# endRouter#This example shows how to verify the configuration:
Router# show spanning-tree summary | include ExtendedExtended system ID is enabled.Configuring the Root Bridge
The switches supported by Cisco IOS Software Release 12.2SX maintain a separate instance of STP for each active VLAN. A bridge ID, consisting of the bridge priority and the bridge MAC address, is associated with each instance. For each VLAN, the network device with the lowest bridge ID becomes the root bridge for that VLAN.
To configure a VLAN instance to become the root bridge, enter the spanning-tree vlan vlan_ID root command to modify the bridge priority from the default value (32768) to a significantly lower value.
When you enter the spanning-tree vlan vlan_ID root command, the switch checks the bridge priority of the current root bridges for each VLAN. With the extended system ID enabled, the switch sets the bridge priority for the specified VLANs to 24576 if this value will cause the switch to become the root for the specified VLANs.
With the extended system ID enabled, if any root bridge for the specified VLANs has a bridge priority lower than 24576, the switch sets the bridge priority for the specified VLANs to 4096 less than the lowest bridge priority. (4096 is the value of the least significant bit of a 4-bit bridge priority value; see Table 22-1.)
Note
With the extended system ID enabled, if all network devices in, for example, VLAN 20 have the default priority of 32768, entering the spanning-tree vlan 20 root primary command on the switch sets the bridge priority to 24576, which causes the switch to become the root bridge for VLAN 20.
Caution
Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of bridge hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically selects an optimal hello time, forward delay time, and maximum age time for a network of that diameter, which can significantly reduce the STP convergence time. You can use the hello keyword to override the automatically calculated hello time.
Note
To configure the switch as the root bridge, perform this task:
Step 1
Router(config)# spanning-tree vlan vlan_ID root primary [diameter hops [hello-time seconds]]
Configures the switch as the root bridge. The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 22-6).
Router(config)# no spanning-tree vlan vlan_ID root
Clears the root bridge configuration.
Step 2
Router(config)# end
Exits configuration mode.
This example shows how to configure the switch as the root bridge for VLAN 10, with a network diameter of 4:
Router# configure terminalRouter(config)# spanning-tree vlan 10 root primary diameter 4Router(config)# endRouter#Configuring a Secondary Root Bridge
When you configure a switch as the secondary root, the STP bridge priority is modified from the default value (32768) so that the switch is likely to become the root bridge for the specified VLANs if the primary root bridge fails (assuming the other network devices in the network use the default bridge priority of 32768).
With the extended system ID is enabled, STP sets the bridge priority to 28672.
You can run this command on more than one switch to configure multiple backup root bridges. Use the same network diameter and hello time values as you used when configuring the primary root bridge.
To configure the switch as the secondary root bridge, perform this task:
Step 1
Router(config)# [no] spanning-tree vlan vlan_ID root secondary [diameter hops [hello-time seconds]]
Configures the switch as the secondary root bridge. The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 17-1 on page 17-2).
Router(config)# no spanning-tree vlan vlan_ID root
Clears the root bridge configuring.
Step 2
Router(config)# end
Exits configuration mode.
This example shows how to configure the switch as the secondary root bridge for VLAN 10, with a network diameter of 4:
Router# configure terminalRouter(config)# spanning-tree vlan 10 root secondary diameter 4Router(config)# endRouter#Configuring STP Port Priority
If a loop occurs, STP considers port priority when selecting a LAN port to put into the forwarding state. You can assign higher priority values to LAN ports that you want STP to select first and lower priority values to LAN ports that you want STP to select last. If all LAN ports have the same priority value, STP puts the LAN port with the lowest LAN port number in the forwarding state and blocks other LAN ports. The possible priority range is 0 through 240 (default 128), configurable in increments of 16.
Cisco IOS 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.
To configure the STP port priority of a Layer 2 LAN interface, perform this task:
Step 1
Router(config)# interface {{gigabitethernet 1/port} | {port-channel port_channel_number}}
Selects an interface to configure.
Step 2
Router(config-if)# spanning-tree port-priority port_priority
Configures the port priority for the LAN interface. The port_priority value can be from 1 to 252 in increments of 4.
Router(config-if)# no spanning-tree port-priority
Reverts to the default port priority value.
Step 3
Router(config-if)# spanning-tree vlan vlan_ID port-priority port_priority
Configures the VLAN port priority for the LAN interface. The port_priority value can be from 1 to 252 in increments of 4. The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 17-1 on page 17-2).
Router(config-if)# [no] spanning-tree vlan vlan_ID port-priority
Reverts to the default VLAN port priority value.
Step 4
Router(config-if)# end
Exits configuration mode.
Step 5
Router# show spanning-tree interface {gigabitethernet 1/port} | {port-channel port_channel_number}
Router# show spanning-tree vlan vlan_ID
Verifies the configuration.
This example shows how to configure the STP port priority of Gigabit Ethernet port 1/4:
Router# configure terminalRouter(config)# interface gigabitethernet 1/4Router(config-if)# spanning-tree port-priority 160Router(config-if)# endRouter#This example shows how to verify the configuration of Gigabit Ethernet port 1/4:
Router# show spanning-tree interface gigabitethernet 1/4Vlan Role Sts Cost Prio.Nbr Status---------------- ---- --- --------- -------- --------------------------------VLAN0001 Back BLK 200000 160.196 P2pVLAN0006 Back BLK 200000 160.196 P2p...VLAN0198 Back BLK 200000 160.196 P2pVLAN0199 Back BLK 200000 160.196 P2pVLAN0200 Back BLK 200000 160.196 P2pRouter#Gigabit Ethernet port 1/4 is a trunk. Several VLANs are configured and active as shown in the example. The port priority configuration applies to all VLANs on this interface.
Note
This example shows how to configure the VLAN port priority of Gigabit Ethernet port 1/4:
Router# configure terminalEnter configuration commands, one per line. End with CNTL/Z.Router(config)# interface gigabitethernet 1/4Router(config-if)# spanning-tree vlan 200 port-priority 64Router(config-if)# endRouter#The configuration entered in the example only applies to VLAN 200. All VLANs other than 200 still have a port priority of 160.
This example shows how to verify the configuration:
Router# show spanning-tree interface gigabitethernet 1/4Vlan Role Sts Cost Prio.Nbr Status---------------- ---- --- --------- -------- --------------------------------VLAN0001 Back BLK 200000 160.196 P2pVLAN0006 Back BLK 200000 160.196 P2p...VLAN0199 Back BLK 200000 160.196 P2pVLAN0200 Desg FWD 200000 64.196 P2pRouter#You also can display spanning tree information for VLAN 200 using the following command:
Router# show spanning-tree vlan 200 interface gigabitethernet 1/4Interface Role Sts Cost Prio.Nbr Status---------------- ---- --- --------- -------- --------------------------------Gi1/4 Desg LRN 200000 64.196 P2pConfiguring STP Port Cost
The STP port path cost default value is determined from the media speed of a LAN interface. If a loop occurs, STP considers port cost when selecting a LAN interface to put into the forwarding state. You can assign 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. The possible cost range is 0 through 200000000 (the default is media specific).
STP uses the port cost value when the LAN interface is configured as an access port and uses VLAN port cost values when the LAN interface is configured as a trunk port.
To configure the STP port cost of a Layer 2 LAN interface, perform this task:
Step 1
Router(config)# interface {{gigabitethernet 1/port} | {port-channel port_channel_number}}
Selects an interface to configure.
Step 2
Router(config-if)# spanning-tree cost port_cost
Configures the port cost for the LAN interface. The port_cost value can be from 1 to 200000000.
Router(config-if)# no spanning-tree cost
Reverts to the default port cost.
Step 3
Router(config-if)# spanning-tree vlan vlan_ID cost port_cost
Configures the VLAN port cost for the LAN interface. The port_cost value can be from 1 to 200000000. The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 17-1 on page 17-2).
Router(config-if)# no spanning-tree vlan vlan_ID cost
Reverts to the default VLAN port cost.
Step 4
Router(config-if)# end
Exits configuration mode.
Step 5
Router# show spanning-tree interface {gigabitethernet 1/port} | {port-channel port_channel_number}
show spanning-tree vlan vlan_ID
Verifies the configuration.
This example shows how to change the STP port cost of Gigabit Ethernet port 1/4:
Router# configure terminalEnter configuration commands, one per line. End with CNTL/Z.Router(config)# interface gigabitethernet 1/4Router(config-if)# spanning-tree cost 1000Router(config-if)# endRouter#This example shows how to verify the configuration:
Router# show spanning-tree interface gigabitethernet 1/4Vlan Role Sts Cost Prio.Nbr Status---------------- ---- --- --------- -------- --------------------------------VLAN0001 Back BLK 1000 160.196 P2pVLAN0006 Back BLK 1000 160.196 P2pVLAN0007 Back BLK 1000 160.196 P2pVLAN0008 Back BLK 1000 160.196 P2pVLAN0009 Back BLK 1000 160.196 P2pVLAN0010 Back BLK 1000 160.196 P2pRouter#This example shows how to configure the port priority at an individual port VLAN cost for VLAN 200:
Router# configure terminalEnter configuration commands, one per line. End with CNTL/Z.Router(config)# interface gigabitethernet 1/4Router(config-if)# spanning-tree vlan 200 cost 2000Router(config-if)# endRouter#This example shows how to verify the configuration:
Router# show spanning-tree vlan 200 interface gigabitethernet 1/4Interface Role Sts Cost Prio.Nbr Status---------------- ---- --- --------- -------- --------------------------------Gi1/4 Desg FWD 2000 64.196 P2p
Note
Router# show spanning-tree vlan 1 interface gigabitethernet 1/4Interface Role Sts Cost Prio.Nbr Status---------------- ---- --- --------- -------- --------------------------------Gi1/4 Back BLK 1000 160.196 P2pRouter#
Note
Configuring the Bridge Priority of a VLAN
Note
To configure the STP bridge priority of a VLAN, perform this task:
Step 1
Router(config)# spanning-tree vlan vlan_ID priority {0 | 4096 | 8192 | 12288 | 16384 | 20480 | 24576 | 28672 | 32768 | 36864 | 40960 | 45056 | 49152 | 53248 | 57344 | 61440}
Configures the bridge priority of a VLAN when the extended system ID is enabled. The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 17-1 on page 17-2).
Router(config)# no spanning-tree vlan vlan_ID priority
Reverts to the default bridge priority value.
Step 2
Router(config)# end
Exits configuration mode.
Step 3
Router# show spanning-tree vlan vlan_ID bridge [detail]
Verifies the configuration.
This example shows how to configure the bridge priority of VLAN 200 to 33792 when the extended system ID is disabled:
Router# configure terminalRouter(config)# spanning-tree vlan 200 priority 32768Router(config)# endRouter#This example shows how to verify the configuration:
Router# show spanning-tree vlan 200 bridgeHello Max FwdVlan Bridge ID Time Age Delay Protocol---------------- -------------------- ---- ---- ----- --------VLAN200 32768 0050.3e8d.64c8 2 20 15 ieeeRouter#Configuring the Hello Time
Note
To configure the STP hello time of a VLAN, perform this task:
Step 1
Router(config)# spanning-tree vlan vlan_ID hello-time hello_time
Configures the hello time of a VLAN. The hello_time value can be from 1 to 10 seconds. The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 17-1 on page 17-2).
Router(config)# no spanning-tree vlan vlan_ID hello-time
Reverts to the default hello time.
Step 2
Router(config)# end
Exits configuration mode.
Step 3
Router# show spanning-tree vlan vlan_ID bridge [detail]
Verifies the configuration.
This example shows how to configure the hello time for VLAN 200 to 7 seconds:
Router# configure terminalRouter(config)# spanning-tree vlan 200 hello-time 7Router(config)# endRouter#This example shows how to verify the configuration:
Router# show spanning-tree vlan 200 bridgeHello Max FwdVlan Bridge ID Time Age Delay Protocol---------------- -------------------- ---- ---- ----- --------VLAN200 49152 0050.3e8d.64c8 7 20 15 ieeeRouter#Configuring the Forward-Delay Time for a VLAN
To configure the STP forward delay time for a VLAN, perform this task:
Step 1
Router(config)# spanning-tree vlan vlan_ID forward-time forward_time
Configures the forward time of a VLAN. The forward_time value can be from 4 to 30 seconds. The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 17-1 on page 17-2).
Router(config)# no spanning-tree vlan vlan_ID forward-time
Reverts to the default forward time.
Step 2
Router(config)# end
Exits configuration mode.
Step 3
Router# show spanning-tree vlan vlan_ID bridge [detail]
Verifies the configuration.
This example shows how to configure the forward delay time for VLAN 200 to 21 seconds:
Router# configure terminalRouter(config)# spanning-tree vlan 200 forward-time 21Router(config)# endRouter#This example shows how to verify the configuration:
Router# show spanning-tree vlan 200 bridgeHello Max FwdVlan Bridge ID Time Age Delay Protocol---------------- -------------------- ---- ---- ----- --------VLAN200 49152 0050.3e8d.64c8 2 20 21 ieeeRouter#Configuring the Maximum Aging Time for a VLAN
To configure the STP maximum aging time for a VLAN, perform this task:
Step 1
Router(config)# spanning-tree vlan vlan_ID max-age max_age
Configures the maximum aging time of a VLAN. The max_age value can be from 6 to 40 seconds. The vlan_ID value can be 1 through 4094, except reserved VLANs (see Table 17-1 on page 17-2).
Router(config)# no spanning-tree vlan vlan_ID max-age
Reverts to the default maximum aging time.
Step 2
Router(config)# end
Exits configuration mode.
Step 3
Router# show spanning-tree vlan vlan_ID bridge [detail]
Verifies the configuration.
This example shows how to configure the maximum aging time for VLAN 200 to 36 seconds:
Router# configure terminalRouter(config)# spanning-tree vlan 200 max-age 36Router(config)# endRouter#This example shows how to verify the configuration:
Router# show spanning-tree vlan 200 bridgeHello Max FwdVlan Bridge ID Time Age Delay Protocol---------------- -------------------- ---- ---- ----- --------VLAN200 49152 0050.3e8d.64c8 2 36 15 ieeeRouter#Enabling Rapid-PVST
Rapid-PVST uses the existing PVST+ framework for configuration and interaction with other features. It also supports some of the PVST+ extensions.
To enable Rapid-PVST mode on the switch, enter the spanning-tree mode rapid-pvst command in privileged mode. To configure the switch in Rapid-PVST mode, see the "Configuring STP" section.
Specifying the Link Type
Rapid connectivity is established only on point-to-point links. Spanning tree views a point-to-point link as a segment connecting only two switches running the spanning tree algorithm. Because the switch assumes that all full-duplex links are point-to-point links and that half-duplex links are shared links, you can avoid explicitly configuring the link type. To configure a specific link type, enter the spanning-tree linktype command.
Restarting Protocol Migration
A switch running both MSTP and RSTP supports a built-in protocol migration process that enables the switch to interoperate with legacy 802.1D switches. If this switch receives a legacy 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MSTP switch can also detect that a port is at the boundary of a region when it receives a legacy BPDU, or an MST BPDU (version 3) associated with a different region, or an RST BPDU (version 2).
However, the switch does not automatically revert to the MSTP mode if it no longer receives 802.1D BPDUs because it cannot determine whether the legacy switch has been removed from the link unless the legacy switch is the designated switch. A switch also might continue to assign a boundary role to a port when the switch to which it is connected has joined the region.
To restart the protocol migration process (force the renegotiation with neighboring switches) on the entire switch, you can use the clear spanning-tree detected-protocols privileged EXEC command. To restart the protocol migration process on a specific interface, enter the clear spanning-tree detected-protocols interface interface-id privileged EXEC command.
Configuring MST
These sections describe how to configure MST:
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Default MST Configuration
Table 22-7 shows the default MST configuration.
MST Configuration Guidelines and Restrictions
When configuring MST, follow these guidelines and restrictions:
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Specifying the MST Region Configuration and Enabling MST
For two or more switches to be in the same MST region, they must have the same VLAN-to-instance mapping, the same configuration revision number, and the same MST name.
A region can have one member or multiple members with the same MST configuration; each member must be capable of processing RSTP BPDUs. There is no limit to the number of MST regions in a network, but each region can only support up to 65 spanning tree instances. You can assign a VLAN to only one spanning tree instance at a time.
To specify the MST region configuration and enable MST, perform this task:
To return to defaults, do the following:
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This example shows how to enter MST configuration mode, map VLANs 10 to 20 to MST instance 1, name the region region1, set the configuration revision to 1, display the pending configuration, apply the changes, and return to global configuration mode:
Router(config)# spanning-tree mst configurationRouter(config-mst)# instance 1 vlan 10-20Router(config-mst)# name region1Router(config-mst)# revision 1Router(config-mst)# show pendingPending MST configurationName [region1]Revision 1Instances configured 2Instance Vlans Mapped-------- ---------------------0 1-9,21-40941 10-20-------------------------------Router(config-mst)# exitRouter(config)#Configuring the Root Bridge
The switch maintains a spanning tree instance for the group of VLANs mapped to it. A switch ID, consisting of the switch priority and the switch MAC address, is associated with each instance. For a group of VLANs, the switch with the lowest switch ID becomes the root bridge.
To configure a switch to become the root bridge, use the spanning-tree mst instance_id root global configuration command to modify the switch priority from the default value (32768) to a significantly lower value so that the switch becomes the root bridge for the specified spanning tree instance. When you enter this command, the switch checks the switch priorities of the root bridges. Because of extended system ID support, the switch sets its own priority for the specified instance to 24576 if this value will cause this switch to become the root bridge for the specified spanning tree instance.
If any root bridge for the specified instance has a switch priority lower than 24576, the switch sets its own priority 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 22-1.)
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 bridge. 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.
The root bridge for each spanning tree instance should be a backbone or distribution switch. Do not configure an access switch as the spanning tree primary root bridge.
Use the diameter keyword, which is available only for MST instance 0, to specify the Layer 2 network diameter (that is, the maximum number of Layer 2 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.
Note
To configure a switch as the root bridge, perform this task:
To return the switch to its default setting, use the no spanning-tree mst instance_id root global configuration command.
Configuring a Secondary Root Bridge
When you configure a switch with the extended system ID support 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 bridge for the specified instance if the primary root bridge fails. This is assuming that the other network switches use the default switch priority of 32768 and therefore are unlikely to become the root bridge.
You can execute this command on more than one switch to configure multiple backup root bridges. Use the same network diameter and hello-time values that you used when you configured the primary root bridge with the spanning-tree mst instance_id root primary global configuration command.
To configure a switch as the secondary root bridge, perform this task:
Step 1
Router# configure terminal
Enters global configuration mode.
Step 2
Router(config)# spanning-tree mst instance_id root secondary [diameter net_diameter [hello-time seconds]]
(Optional) Configures a switch as the secondary root bridge.
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Use the same network diameter and hello-time values that you used when configuring the primary root bridge. See the "Configuring the Root Bridge" section.
Step 3
Router(config)# end
Returns to privileged EXEC mode.
Step 4
Router# show spanning-tree mst instance_id
Verifies your entries.
Step 5
Router# copy running-config startup-config
(Optional) Saves your entries in the configuration file.
To return the switch to its default setting, use the no spanning-tree mst instance_id root global configuration command.
Configuring Port Priority
If a loop occurs, MST uses the port priority when selecting an interface to put into the forwarding state. You can assign higher priority values (lower numerical values) to interfaces that you want selected first and lower priority values (higher numerical values) that you want selected last. If all interfaces have the same priority value, MST puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.
To configure the MST port priority of an interface, perform this task:
Note
To return the interface to its default setting, use the no spanning-tree mst instance_id port-priority interface configuration command.
Configuring Path Cost
The MST path cost default value is derived from the media speed of an interface. If a loop occurs, MST 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 interfaces have the same cost value, MST puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.
To configure the MST cost of an interface, perform this task:
Note
To return the interface to its default setting, use the no spanning-tree mst instance_id cost interface configuration command.
Configuring the Switch Priority
You can configure the switch priority so that it is more likely that a switch is chosen as the root bridge.
Note
To configure the switch priority, perform this task:
To return the switch to its default setting, use the no spanning-tree mst instance_id priority global configuration command.
Configuring the Hello Time
You can configure the interval between the generation of configuration messages by the root bridge by changing the hello time.
Note
To configure the hello time for all MST instances, perform this task:
To return the switch to its default setting, use the no spanning-tree mst hello-time global configuration command.
Configuring the Forwarding-Delay Time
To configure the forwarding-delay time for all MST instances, perform this task:
To return the switch to its default setting, use the no spanning-tree mst forward-time global configuration command.
Configuring the Transmit Hold Count
To configure the transmit hold count for all MST instances, perform this task:
To return the switch to its default setting, use the no spanning-tree transmit hold-count global configuration command.
Configuring the Maximum-Aging Time
To configure the maximum-aging time for all MST instances, perform this task:
To return the switch to its default setting, use the no spanning-tree mst max-age global configuration command.
Configuring the Maximum-Hop Count
To configure the maximum-hop count for all MST instances, perform this task:
To return the switch to its default setting, use the no spanning-tree mst max-hops global configuration command.
Specifying the Link Type to Ensure Rapid Transitions
If you connect a port to another port through a point-to-point link and the local port becomes a designated port, the RSTP negotiates a rapid transition with the other port by using the proposal-agreement handshake to ensure a loop-free topology as described in the "Rapid Convergence" section.
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 switch running MST, you can override the default setting of the link type and enable rapid transitions to the forwarding state.
To override the default link-type setting, perform this task:
To return the port to its default setting, use the no spanning-tree link-type interface configuration command.
Designating the Neighbor Type
A topology could contain both prestandard and 802.1s standard compliant devices. By default, ports can automatically detect prestandard devices, but they can still receive both standard and prestandard BPDUs. When there is a mismatch between a device and its neighbor, only the CIST runs on the interface.
You can choose to set a port to send only prestandard BPDUs. The prestandard flag appears in all the show commands, even if the port is in STP compatibility mode.
To override the default link-type setting, perform this task:
To return the port to its default setting, use the no spanning-tree mst prestandard interface configuration command.
Restarting the Protocol Migration Process
A switch running MST supports a built-in protocol migration feature that enables it to interoperate with 802.1D switches. If this switch receives an 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MST switch also can detect that a port is at the boundary of a region when it receives an 802.1D BPDU, an MST BPDU (Version 3) associated with a different region, or an RST BPDU (Version 2).
However, the switch does not automatically revert to the MST mode if it no longer receives 802.1D BPDUs because it cannot detect whether the 802.1D switch has been removed from the link unless the 802.1D switch is the designated switch. A switch also might continue to assign a boundary role to a port when the switch to which it is connected has joined the region.
To restart the protocol migration process (force the renegotiation with neighboring switches) on the switch, use the clear spanning-tree detected-protocols privileged EXEC command.
To restart the protocol migration process on a specific interface, use the clear spanning-tree detected-protocols interface interface_id privileged EXEC command.
Displaying the MST Configuration and Status
To display the spanning-tree status, use one or more of the privileged EXEC commands that are described in Table 22-8.
