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Catalyst 4000 Family Software Configuration Guide, 7.1
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Index
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Preface
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Product Overview
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Using the Command-Line Interface
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Configuring the Switch IP Address and Default Gateway
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Configuring Ethernet and Fast Ethernet Switching
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Configuring Gigabit Ethernet Switching
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Configuring Fast EtherChannel and Gigabit EtherChannel
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Configuring Spanning Tree
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Configuring Spanning-Tree PortFast, UplinkFast, and BackboneFast
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Configuring VTP
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Configuring VLANs
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VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports
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Configuring Dynamic Port VLAN Membership with VMPS
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Configuring GVRP
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Configuring Quality of Service
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Configuring Multicast Services
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Configuring Port Security
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Configuring the IP Permit List
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Configuring Protocol Filtering
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Checking Port Status and Connectivity
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Configuring CDP
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Using Switch TopN Reports
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Configuring UDLD
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Configuring SNMP
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Configuring RMON
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Configuring SPAN
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Administering the Switch
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Switch Access: Using Authentication, Authorization, and Accounting
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Modifying the Switch Boot Configuration
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Working with System Software Images
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Using the Flash File System
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Working with Configuration Files
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Configuring Switch Acceleration
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Configuring System Message Logging
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Configurinng DNS
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Configuring NTP
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Acronyms
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Index
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Table Of Contents
How Spanning Tree Protocols Work
How a Switch or Port Becomes the Root Switch or Root Port
How Bridge Protocol Data Units Work
Calculating and Assigning Port Costs
Calculating the Port Cost Using the Short Method
Calculating the Port Cost Using the Long Method
Calculating the Port Cost for Aggregate Links
Understanding PVST+ and MISTP Modes
Understanding How Multiple Spanning Trees Work
Configuring PVST+ Bridge ID Priority
Configuring PVST+ Port Priority
Configuring PVST+ Default Port Cost Mode
Configuring PVST+ Port VLAN Cost
Configuring PVST+ Port VLAN Priority
Disabling the PVST+ Mode on a VLAN
Configuring MISTP Bridge ID Priority
Configuring MISTP Port Priority
Configuring MISTP Port Instance Cost
Configuring MISTP Port Instance Priority
Mapping VLANs to a MISTP Instance
Determining MISTP Instance—VLAN Mapping Conflicts
Unmapping VLANs from a MISTP Instance
Disabling MISTP-PVST+ or MISTP
Configuring a Primary Root Switch
Configuring a Secondary Root Switch
Configuring a Root Switch to Improve Convergence
Using Root Guard—Preventing Switches from Becoming Root
Configuring Spanning Tree Timers
Configuring Forward Delay Time
Configuring Maximum Aging Time
Understanding How BPDU Skewing Works
Configuring Spanning Tree BPDU Skewing
Configuring Multiple Spanning Tree
Enabling Multiple Spanning Tree
Configuring the MST Bridge ID Priority
Configuring the MST Port Priority
Configuring the MST Port Instance Cost
Configuring the MST Port Instance Priority
Mapping and Unmapping VLANs to an MST Instance
Configuring Spanning Tree
This chapter provides a brief overview of the IEEE 802.1D bridge Spanning Tree Protocol (STP) and describes how to use and configure Cisco's proprietary Spanning Tree Protocols, Per VLAN Spanning Tree + (PVST+), and Multi-Instance Spanning Tree Protocol (MISTP) on the Catalyst enterprise LAN switches.
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For information on configuring the spanning tree PortFast, UplinkFast, and BackboneFast features, see "Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast, and Loop Guard."
This chapter consists of these sections:
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How Spanning Tree Protocols Work
This section describes the specific functions that are common to all spanning tree protocols. Cisco's proprietary spanning tree protocols, PVST+ and MISTP, are based on the IEEE 802.1D STP. (See the "Understanding PVST+ and MISTP Modes" section for information about PVST+ and MISTP.) The 802.1D STP is a Layer 2 management protocol that provides path redundancy in a network while preventing undesirable loops. All spanning tree protocols use an algorithm that calculates the best loop-free path through the network.
STP uses a distributed algorithm that selects one bridge of a redundantly connected network as the root of a spanning tree connected active topology. STP assigns roles to each port depending on what the port's function is in the active topology. Port roles are as follows:
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Switches that have ports with these assigned roles are called root or designated switches. See the next section, How a Topology Is Created.
In Ethernet networks, only one active path may exist between any two stations. Multiple active paths between stations can cause loops in the network. When loops occur, some switches recognize stations on both sides of the switch. This situation causes the forwarding algorithm to malfunction allowing duplicate frames to be forwarded.
Spanning tree algorithms provide path redundancy by defining a tree that spans all of the switches in an extended network and then forces certain redundant data paths into a standby (blocked) state. At regular intervals the switches in the network send and receive spanning tree packets which they use to identify the active path. If one network segment becomes unreachable, or if spanning tree costs change, the spanning tree algorithm reconfigures the spanning tree topology and reestablishes the link by activating a standby path.
Spanning tree operation is transparent to end stations, which do not detect whether they are connected to a single LAN segment or a switched LAN of multiple segments.
How a Topology Is Created
All switches in an extended LAN participating in a spanning tree gather information about other switches in the network through an exchange of data messages known as bridge protocol data units (BPDUs). This exchange of messages results in the following actions:
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The topology of 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. A spanning tree protocol uses BPDUs to elect the root switch and root port for the switched network, as well as the root port and designated port for each switched segment.
How a Switch or Port Becomes the Root Switch or Root Port
If all switches in a network are enabled with default settings, the switch with the lowest MAC address becomes the root switch. In the network shown in Figure 7-1, Switch A, with the lowest MAC address, is the root switch. However, due to traffic patterns, number of forwarding ports, or line types, Switch A might not be the ideal root switch. A switch can be forced to become the root switch by increasing the priority (that is, lowering the priority number) on the preferred switch. This causes the spanning tree to recalculate the topology and make the selected switch the root switch.
Figure 7-1 Configuring a Loop-Free Topology
You can also change the priority of a port in order to make it the root port. When the spanning tree topology is based on default parameters, the path between source and destination stations in a switched network might not be ideal. The goal is to make the fastest link the root port, connecting higher-speed links to a port that has a higher number than the current root port can cause a root-port change.
For example, assume that a port on Switch B is a fiber-optic link. Also, 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 Port Priority parameter for the UTP port to a higher priority (lower numerical value) than the fiber-optic port, the UTP port becomes the root port. You could also accomplish this scenario by changing the Port Cost parameter for the UTP port to a lower value than that of the fiber-optic port.
How Bridge Protocol Data Units Work
BPDUs contain configuration information about the transmitting switch and its ports, including switch and port MAC addresses, switch priority, port priority, and port cost. Each configuration BPDU contains this information:
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The switch sends configuration BPDUs to communicate with and compute the spanning tree topology. A MAC frame conveying a BPDU sends the switch group address to the destination address field. All switches connected to the LAN on which the frame is transmitted receive the BPDU. BPDUs are not directly forwarded by the switch, but the receiving switch uses the information in the frame to calculate a BPDU and, if the topology changes, initiates a BPDU transmission.
A BPDU exchange results in the following:
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Calculating and Assigning Port Costs
By calculating and assigning the port cost of the switch ports, you can ensure that the shortest (lowest cost) distance to the root switch is used to transmit data. You can calculate and assign lower path cost values (port costs) to higher bandwidth ports by using either the short method (which is the default) or the long method. The short method uses a 16-bit format that yields values from 1 to 65535. The long method uses a 32-bit format that yields values in the range of 1 to 200,000,000. For more information on setting the default cost mode, see the "Configuring PVST+ Default Port Cost Mode" section.
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Calculating the Port Cost Using the Short Method
The IEEE 802.1D specification assigns 16-bit (short) default port cost values to each port that is based on bandwidth. You can also manually assign port costs between 1-65535. The 16-bit values are only used for ports that have not been specifically configured for port cost. Table 7-1 shows the default port cost values that are assigned by the switch for each type of port when you use the short method to calculate the port cost.
Table 7-1 Default Port Cost Values Using the Short Method
10 Mbps
100
1 to 65535
100 Mbps
19
1 to 65535
1 Gbps
4
1 to 65535
Calculating the Port Cost Using the Long Method
802.1t assigns 32-bit (long) default port cost values to each port using a formula that is based on the bandwidth of the port. You can also manually assign port costs between 1-200,000,000. The formula for obtaining default 32-bit port costs is to divide the bandwidth of the port by 200,000,000. Table 7-2 shows the default port cost values that are assigned by the switch and the recommended cost values and ranges for each type of port when you use the long method to calculate port cost.
Calculating the Port Cost for Aggregate Links
As individual links are added or removed from an aggregate link (port bundle), the bandwidth of the aggregate link increases or decreases. These changes in bandwidth lead to recalculation of the default port cost for the aggregated port. Changes to the default port cost or changes resulting from links that autonegotiate their bandwidth could lead to recalculation of the spanning tree topology which may not be desirable, especially if the added or removed link is of little consequence to the bandwidth of the aggregate link (for example, if a 10-Mbps link is removed from a 10-Gbps aggregate link). Because of the limitations that are presented by automatically recalculating the topology, 802.1t states that changes in bandwidth will not result in changes to the cost of the port. Therefore, the aggregated port will use the same port cost parameters as a standalone port.
Spanning Tree Port States
Topology changes can take place in a switched network due to a link coming up or going down (failing). When a switch port transitions directly from nonparticipation in the topology to the forwarding state, it can create temporary data loops. Ports must wait for new topology information to propagate through the switches in the LAN before they can start forwarding frames. They must also allow the frame lifetime to expire for frames that have been forwarded using the old topology.
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At any given time each port on a switch using STP is in one of these states:
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A port moves through these states:
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Figure 7-2 illustrates how a port moves through the states.
Figure 7-2 STP Port States
You can modify each port state by using management software, such as VLAN Trunk Protocol (VTP). When you enable spanning tree, every switch in the network goes through the blocking state and the transitory states of listening and learning at power up. If properly configured, each port stabilizes into the forwarding or blocking state.
When the spanning tree algorithm places a port in the forwarding state, the following occurs:
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Blocking State
A port in the blocking state, such as port 2 in Figure 7-3, does not participate in frame forwarding. After initialization a BPDU is sent to each port in the switch. A switch initially assumes it is the root until it exchanges BPDUs with other switches. This exchange establishes which switch in the network is really the root. If only one switch resides in the network, no exchange occurs, the forward delay timer expires, and the ports move to the listening state. A switch always enters the blocking state following switch initialization.
Figure 7-3 Port 2 in Blocking State
A port in the blocking state performs as follows:
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Listening State
The listening state is the first transitional state a port enters after the blocking state. The port enters this state when the spanning tree determines that the port should participate in frame forwarding. Learning is disabled in the listening state. Figure 7-4 shows a port in the listening state.
Figure 7-4 Port 2 in Listening State
A port in the listening state performs as follows:
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Learning State
A port in the learning state prepares to participate in frame forwarding. The port enters the learning state from the listening state. Figure 7-5 shows a port in the learning state.
Figure 7-5 Port 2 in Learning State
A port in the learning state performs as follows:
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Forwarding State
A port in the forwarding state forwards frames, as shown in Figure 7-6. The port enters the forwarding state from the learning state.
Figure 7-6 Port 2 in Forwarding State
A port in the forwarding state performs as follows:
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Disabled State
A port in the disabled state does not participate in frame forwarding or STP, as shown in Figure 7-7. A port in the disabled state is virtually nonoperational.
Figure 7-7 Port 2 in Disabled State
A disabled port performs as follows:
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Understanding PVST+ and MISTP Modes
Catalyst 4000 family switches provide two proprietary spanning tree modes based on the IEEE 802.1D standard and one mode that is a combination of the two modes:
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An overview of each mode is provided in this section. Each mode is described in detail in these sections:
Caution
PVST+ Mode
PVST+ is the default Spanning Tree Protocol used on all Ethernet, Fast Ethernet, and Gigabit Ethernet port-based VLANs on Catalyst 4000 family switches. PVST+ runs on each VLAN on the switch, ensuring that each has a loop-free path through the network.
PVST+ provides Layer 2 load balancing for the VLAN on which it runs; you can create different logical topologies using the VLANs on your network to ensure that all of your links will be used but no one link will be oversubscribed.
Each instance of PVST+ on a VLAN has a single root switch. This root switch propagates the spanning tree information associated with that VLAN to all other switches in the network. Because each switch has the same knowledge about the network, this process ensures that the network topology is maintained.
MISTP Mode
MISTP is an optional spanning tree protocol that runs on Catalyst 4000 family switches. MISTP allows you to group multiple VLANs under a single instance of spanning tree (a MISTP instance). MISTP combines the Layer 2 load-balancing benefits of PVST+ with the lower CPU load of IEEE 802.1Q.
A MISTP instance is a virtual logical topology defined by a set of bridge and port parameters; a MISTP instance becomes a real topology when VLANs are mapped to it. Each MISTP instance has its own root switch and a different set of forwarding links (that is different bridge and port parameters).
Each instance of MISTP has a single root switch. This root switch propagates the information associated with that instance of MISTP to all other switches in the network. This process ensures that the network topology is maintained because each switch has the same knowledge about the network.
MISTP builds MISTP instances by exchanging MISTP BPDUs with peer entities in the network. There is only one BPDU for each MISTP instance, rather than for each VLAN as in PVST+. There are fewer BPDUs in a MISTP network; therefore, there is less overhead in the network. MISTP discards any PVST+ BPDUs that it sees.
A MISTP instance can have any number of VLANs mapped to it, but a VLAN can only be mapped to a single MISTP instance. You can easily move a VLAN (or VLANs) in a MISTP topology to another MISTP instance if it has converged. (However, if ports are added at the same time the VLAN is moved, convergence time is required.)
MISTP-PVST+ Mode
MISTP-PVST+ is a transition spanning tree mode that allows you to use the MISTP functionality on Catalyst 4000 family switches while continuing to communicate with the older Catalyst 5000 and 6000 switches in your network that use PVST+. A switch using PVST+ mode and a switch using MISTP mode connected together cannot see the BPDUs of the other switch, a condition that can cause loops in the network. MISTP-PVST+ allows interoperability between PVST+ and pure MISTP, because it detects the BPDUs of both modes. If you wish to convert your network to MISTP, you can use MISTP-PVST+ to transition the network from PVST+ to MISTP in order to avoid problems.
MISTP-PVST+ conforms to the limits of PVST+; for example, you can only configure the amount of VLAN ports on your MISTP-PVST+ switches that you configure on your PVST+ switches.
Bridge Identifiers
The follwoing section explains how MAC addresses are used in PVST+ and MISTP as unique bridge identifiers:
MAC Address Allocation
Catalyst 4000 family switches have a pool of 1024 MAC addresses that can be used as bridge identifiers for VLANs running under PVST+ or for MISTP instances. You can use the show module command to view the MAC address range.
MAC addresses are allocated sequentially, with the first MAC address in the range assigned to VLAN 1, the second in the range assigned to VLAN 2, and so forth. The last MAC address in the range is assigned to the supervisor engine in-band (sc0) management interface.
For example, if the MAC address range for the supervisor engine is 00-e0-1e-9b-2e-00 to 00-e0-1e-9b-31-ff, the VLAN 1 bridge ID is 00-e0-1e-9b-2e-00, the VLAN 2 bridge ID is 00-e0-1e-9b-2e-01, the VLAN 3 bridge ID is 00-e0-1e-9b-2e-02, and so forth. The in-band (sc0) interface MAC address is 00-e0-1e-9b-31-ff.
MAC Address Reduction
The MAC address reduction feature is used on Catalyst 6000 family switches to enable extended-range VLAN identification. If you have a Catalyst 6000 switch in your network and you have MAC address reduction enabled on it, you should also enable MAC address reduction on all your Catalyst 4000 family switches to avoid problems in the spanning tree topology. When MAC address reduction is enabled on Catalyst 4000 family switches, it disables the pool of MAC addresses used for the VLAN spanning tree, leaving a single MAC address that identifies the switch. For detailed information on the MAC address reduction feature, refer to the Catalyst 6000 Software Configuration Guide.
Understanding How Multiple Spanning Trees Work
The Multiple Spanning Tree (MST) feature is an upcoming (as of this software release) IEEE standard. 802.1s for MST is an amendment to 802.1Q. MST extends the 802.1w Rapid Spanning Tree (RST) algorithm to multiple spanning trees. This extension provides for both rapid convergence and load balancing in a VLAN environment. The MST protocol is currently being further developed and the MST feature for this release is based on a draft version of the IEEE standard. The protocol as implemented in this release is backward compatible with 802.1D STP, 802.1w the Rapid Spanning Tree Protocol (RSTP), and the Cisco PVST+ architecture.
MST allows you to build multiple spanning trees over VLAN trunks. You can group and associate VLANs to spanning tree instances. Each instance can have a topology independent of other spanning tree instances. This new architecture provides multiple forwarding paths for data traffic and enables load balancing. Network fault tolerance is improved because a failure in one instance (forwarding path) does not affect other instances (forwarding paths).
In large networks, having different VLAN-spanning tree instance assignments located in different parts of the network makes it easier to administrate and optimally utilize redundant paths. However, a spanning tree instance can exist only on bridges that have compatible VLAN-instance assignments. Therefore, MST requires that you configure a set of bridges with the same MST configuration information, allowing them to participate in a given set of spanning tree instances. Interconnected bridges that have the same MST configuration are referred to as an MST region.
MST uses the modified RSTP version called the Multiple Spanning Tree Protocol (MSTP). The MST feature has these characteristics:
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M-records are always encapsulated within MST BPDUs (MST BPDUs). The original spanning trees computed by MSTP are called M-trees. M-trees are active only within the MST region. M-trees merge with the IST at the boundary of the MST region and form the CST.
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For MST to operate correctly, the following apply:
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These sections describe the MST feature:
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Rapid Spanning Tree Protocol
RSTP significantly reduces the time it takes to reconfigure the active topology of the network when changes to the physical topology or its configurations parameters occur. RSTP selects one switch as the root of a spanning-tree-connected active topology and assigns port roles to individual ports of the switch, depending on whether that port is part of the active topology.
RSTP provides rapid connectivity following the failure of a switch, switch port, or a LAN. A new root port and the designated port on the other side of the bridge transition to forwarding through an explicit handshake between them. RSTP allows switch port configuration so the ports can transition to forwarding directly when the switch reinitializes.
RSTP, specified in 802.1w, supersedes STP specified in 802.1D, while retaining compatibility with STP. RSTP provides the structure on which the MST operates. You configure RSTP when you configure the MST feature. For more information, see the "Configuring Multiple Spanning Tree" section.
RSTP provides backward compatibility with 802.1D bridges, as follows:
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RSTP Port Roles
RSTP uses the following definitions for port roles:
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Port roles are assigned as follows:
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RSTP Port States
The port state controls the forwarding and learning processes and provides the values of discarding, learning, and forwarding. Table 7-3 provides a comparison between STP port states and RSTP port states.
Table 7-3 Comparison Between STP and RSTP Port States
Enabled
Blocking1
Discarding2
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Listening
Discarding
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Learning
Learning
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Forwarding
Forwarding
Yes
Disabled
Disabled
Discarding
No
1 IEEE 802.1D port state designation.
2 IEEE 802.1w port state designation.Discarding is analogous with, and the same as blocking in MST in this document.
In a stable topology, RSTP ensures that every root port and designated port transition to forwarding while all alternate ports and backup ports are always in the discarding state.
MST-to-SST Interoperability
A virtual bridged LAN may contain interconnected regions of SST and MST bridges. Figure 7-8 shows this relationship.
Figure 7-8 Network with Interconnected SST and MST Regions
To the spanning tree protocol running in the SST region, an MST region appears as a single SST or pseudobridge. Pseudobridges operate as follows:
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Common Spanning Tree
802.1Q specifies a single spanning tree for all the VLANs called CST. In a Catalyst 4000 family switch running PVST+ the VLAN 1 spanning tree corresponds to CST, in a Catalyst 4000 family switch running MST, IST (instance 0) corresponds to CST.
MST Instances
This release supports up to 16 instances; each spanning tree instance is identified by an instance ID that ranges from 0 to 15. Instance 0 is mandatory and is always present. Instances 1 through 15 are optional.
MST Configuration
MST configuration has three parts as follows:
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You must configure each byte manually. You can use SNMP or the CLI to perform the configuration.
MST BPDUs contain the MST configuration ID and the checksum. An MST bridge accepts an MST BPDU only if the MST BPDU configuration ID and the checksum match its own MST region configuration ID and checksum. If one value is different, the MST BPDU is treated as an SST BPDU.
When you modify an MST configuration through either a console or Telnet connection, the session exits without committing those changes and the edit buffer locks. Further configuration is impossible until you discard the existing edit buffer and acquire a new edit buffer by entering the set spantree mst config rollback force command.
MST Region
Interconnected bridges that have the same MST configuration are referred to as an MST region. There is no limit on the number of MST regions in the network.
To form an MST region, bridges can be either of the following:
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If you connect two MST regions with different MST configurations, the MST regions do the following:
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Boundary Ports
A port that connects an MST region to an SST region running RSTP (802.1w), an SST region running STP (802.1D), or another MST region is a boundary port. A boundary port is a port that connects to a LAN, the designated bridge of which, is either an SST bridge or a bridge with a different MST configuration. A designated port knows that it is on the boundary if it detects an STP bridge, or receives an agreement message from an RST or MST bridge with a different configuration.
At the boundary, the role of MST ports do not matter; their state is forced to be the same as the IST port state. If the boundary flag is set for the port, the MSTP Port Role selection mechanism assigns a port role to the boundary and the same state as that of the IST port. The IST port at the boundary can take up any port role except a backup port role.
IST Master
The IST master of an MST region is the bridge with the lowest bridge identifier and the least path cost to the CST root. If an MST bridge is the root bridge for CST, then it is the IST master of that MST region. If the CST root is out side the MST region, then one of the MST bridges at the boundary is selected as the IST master. Other bridges on the boundary that belong to the same region eventually block the boundary ports that lead to the root.
If two or more bridges at the boundary of the region have an identical path to the root, you can set a slightly lower bridge priority to make a specific bridge IST master.
The root path cost and message age inside a region stays constant, but the IST path cost is incremented and the IST remaining hops is decremented at each hop. Enter the show spantree mst command to display the information about the IST master, path cos, and remaining hops for the bridge.
Edge Ports
A port that is connected to a nonbridging device (for example a host or a router) is an edge port. A port that connects to a hub is also an edge port, provided that the hub or any LAN that is connected by it does not have a bridge. These ports start forwarding as soon as the link is up.
MST requires that all ports are configured for each host or router. To establish rapid connectivity after a failure you need to block nonedge designated ports of an intermediate bridge. If the port connects to another bridge that can send back an agreement, then the port starts forwarding immediately. Otherwise, twice the forward delay time is needed for that port to start forwarding again. Explicitly configuring the ports that are connected to the hosts and routers as edge ports is essential while using MST.
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To prevent a misconfiguration PortFast is turned off operationally if the port receives a BPDU. You can display the configured and operational status of Portfast by using the show spantree mst mod/port command.
Link Type
Rapid connectivity is established only on point-to-point links. For correct operation of the protocol, ports must be explicitly configured to a host or router. However, cabling in most networks meets this requirement, and explicit configuration is avoided by treating all full-duplex links as point-to-point links. Enter the set spantree mst link-type command to configure point-to-point links.
Message Age and Hop Count
IST and MST instances do not use the Message Age and Maximum Age timer settings in the BPDU. IST and MST use a separate hop count mechanism that is very similar to the IP TTL mechanism. You can configure each MST bridge with a maximum hop count. The root bridge of the instance sends a BPDU (or M-record) with the remaining hop count that is equal to the maximum hop count. When a bridge receives a BPDU (or M-record), it decrements the received remaining hop count by one. The bridge discards the BPDU (M-record) and ages out the information held for the port if the count reaches zero after decrementing. The nonroot bridges propagate the decremented count as the remaining hop count in the BPDUs (M-records) they generate.
The Message Age and Maximum Age timer settings in the RST portion of the BPDU remain the same throughout the region, and the same values are propagated by the region's designated ports at the boundary.
MST-to-PVST+ Interoperability
These guidelines apply in a topology where you configure MST switches (all in the same region) to interact with PVST+ switches that have VLANs 1—100 set up to span throughout the network:
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Console> (enable) show spantree mst 3Spanning tree mode MSTInstance 3VLANs Mapped: 31-40Designated Root 00-10-7b-bb-2f-00Designated Root Priority 8195 (root priority:8192, sys ID ext:3)Designated Root Cost 0 Remaining Hops 20Designated Root Port 1/0Bridge ID MAC ADDR 00-10-7b-bb-2f-00Bridge ID Priority 8195 (bridge priority:8192, sys ID ext:3)Port State Role Cost Prio Type------------------------ ------------- ---- -------- ------------------------6/1 forwarding BDRY 10000 30 P2P,Boundary(PVST)6/2 blocking BDRY 20000 32 P2P,Boundary(PVST)If you enable loop guard on the PVST+ switches, the ports might change to a loop-inconsistent state when the MST switches change their configuration. To correct the loop-inconsistent state, you must disable and reenable loop guard on that PVST+ switch.
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When you connect a PVST+ switch to two different MST regions, the topology change from the PVST+ switch does not pass beyond the first MST region. In this case, the topology changes are only propagated in the instance to which the VLAN is mapped. The topology change stays local to the first MST region and the CAM entries in the other region are not flushed To make the topology change visible throughout other MST regions, you can map that VLAN to IST or connect the PVST+ switch to the two regions through access links.
Using PVST+
PVST+ is the default spanning tree mode for Catalyst 4000 family switches. These sections describe how to configure PVST+ on Ethernet VLANs:
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Default PVST+ Configuration
Table 7-4 shows the default PVST+ configuration.
Configuring PVST+ Bridge ID Priority
The bridge ID priority is the priority of a VLAN when the switch is in PVST+ mode.
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The bridge priority is combined with the system ID extension (that is, the ID of the VLAN) to create the bridge ID priority for the VLAN.
To configure the spanning tree bridge priority for a VLAN, perform this task in privileged mode:
Step 1
Set the bridge ID priority for a VLAN.
set spantree priority bridge_ID_priority [vlan]
Step 2
Verify the bridge ID priority.
show spantree [vlan] [active]
This example shows the bridge ID when MAC address reduction is not enabled (default):
Console> (enable) set spantree priority 30000 1Spantree 1 bridge priority set to 30000.Console> (enable) show spantree 1VLAN 1Spanning tree mode PVST+Spanning tree type ieeeSpanning tree enabledDesignated Root 00-60-70-4c-70-00Designated Root Priority 16384Designated Root Cost 19Designated Root Port 2/3Root Max Age 14 sec Hello Time 2 sec Forward Delay 10 secBridge ID MAC ADDR 00-d0-00-4c-18-00Bridge ID Priority 30000Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 secPort Vlan Port-State Cost Prio Portfast Channel_id------------------------ ---- ------------- --------- ---- -------- ----------1/1 1 not-connected 4 32 disabled 01/2 1 not-connected 4 32 disabled 02/1 1 not-connected 100 32 disabled 02/2 1 not-connected 100 32 disabled 0This example shows the bridge ID priority when MAC reduction is enabled:
Console> (enable) set spantree priority 32768 1Spantree 1 bridge ID priority set to 32769(bridge priority: 32768 + sys ID extension: 1)Console> (enable) show spantree 1/1 1VLAN 1Spanning tree mode PVST+Spanning tree type ieeeSpanning tree enabledDesignated Root 00-60-70-4c-70-00Designated Root Priority 16384Designated Root Cost 19Designated Root Port 2/3Root Max Age 14 sec Hello Time 2 sec Forward Delay 10 secBridge ID MAC ADDR 00-d0-00-4c-18-00Bridge ID Priority 32769 (bridge priority: 32768, sys ID ext: 1)Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 secPort Vlan Port-State Cost Prio Portfast Channel_id------------------------ ---- ------------- --------- ---- -------- ----------1/1 1 not-connected 4 32 disabled 01/2 1 not-connected 4 32 disabled 02/1 1 not-connected 100 32 disabled 02/2 1 not-connected 100 32 disabled 0Configuring PVST+ Port Cost
You can configure the port cost of switch ports. Ports with lower port costs are more likely to be chosen to forward frames. Assign lower numbers to ports attached to faster media (such as full duplex) and higher numbers to ports attached to slower media.The possible range of cost is 1 to 65535. The default differs for different media. Path cost is typically 1000 ÷ LAN speed in megabits per second.
To configure the port cost for a port, perform this task in privileged mode:
Step 1
Configure the port cost for a switch port.
set spantree portcost {mod/port} cost
Step 2
Verify the port cost setting.
show spantree mod/port
This example shows how to configure the port VLAN priority on a port and verify the configuration:
Console> (enable) set spantree portcost 2/3 12Spantree port 2/3 path cost set to 12.Console> (enable) show spantree 2/3
