Spanning Tree Configuration (3.3)
In this section, you will learn how to implement PVST+ and Rapid PVST+ in a switched LAN environment.
PVST+ Configuration (3.3.1)
The focus of this topic is on how to configure PVST+ in a switched LAN environment.
Catalyst 2960 Default Configuration (3.3.1.1)
Table 3-7 shows the default spanning-tree configuration for a Cisco Catalyst 2960 Series switch. Notice that the default spanning-tree mode is PVST+.
Table 3-7 Default Switch Configuration
| Feature | Default Setting |
|---|---|
| Enable state | Enabled on VLAN 1 |
| Spanning-tree mode | PVST+ (Rapid PVST+ and MSTP are disabled.) |
| Switch priority | 32768 |
| Spanning-tree port priority (configurable on a per-interface basis) | 128 |
| Spanning-tree port cost (configurable on a per-interface basis) | 1000 Mbps: 4 |
| 100 Mbps: 19 | |
| 10 Mbps: 100 | |
| Spanning-tree VLAN port priority (configurable on a per-VLAN basis) | 128 |
| Spanning-tree VLAN port cost (configurable on a per-VLAN basis) | 1000 Mbps: 4 |
| 100 Mbps: 19 | |
| 10 Mbps: 100 | |
| Spanning-tree timers | Hello time: 2 seconds |
| Forward-delay time: 15 seconds | |
| Maximum-aging time: 20 seconds | |
| Transmit hold count: 6 BPDUs |
Configuring and Verifying the Bridge ID (3.3.1.2)
When an administrator wants a specific switch to become a root bridge, the bridge priority value must be adjusted to ensure that it is lower than the bridge priority values of all the other switches on the network. There are two different methods to configure the bridge priority value on a Cisco Catalyst switch.
Method 1
To ensure that a switch has the lowest bridge priority value, use the spanning-tree vlan vlan-id root primary command in global configuration mode. The priority for the switch is set to the predefined value of 24,576 or to the highest multiple of 4096 less than the lowest bridge priority detected on the network.
If an alternate root bridge is desired, use the spanning-tree vlan vlan-id root secondary global configuration mode command. This command sets the priority for the switch to the predefined value 28,672. This ensures that the alternate switch becomes the root bridge if the primary root bridge fails. This assumes that the rest of the switches in the network have the default 32,768 priority value defined.
In Figure 3-39, S1 has been assigned as the primary root bridge, using the spanning-tree vlan 1 root primary command, and S2 has been configured as the secondary root bridge, using the spanning-tree vlan 1 root secondary command.
Method 2
Another method for configuring the bridge priority value is by using the spanning-tree vlan vlan-id priority value global configuration mode command. This command gives more granular control over the bridge priority value. The priority value is configured in increments of 4096 between 0 and 61,440.
In the example in Figure 3-39, S3 has been assigned a bridge priority value of 24,576, using the spanning-tree vlan 1 priority 24576 command.
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Figure 3-39 Configuring the Bridge ID
To verify the bridge priority of a switch, use the show spanning-tree command. In Example 3-4, the priority of the switch has been set to 24,576. Also notice that the switch is designated as the root bridge for the spanning-tree instance.
Example 3-4 Verifying the Root Bridge and BID
S3# show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 000A.0033.0033
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
Address 000A.0033.3333
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- ------------------------
Fa0/1 Desg FWD 4 128.1 P2p
Fa0/2 Desg FWD 4 128.2 P2p
PortFast and BPDU Guard (3.3.1.3)
PortFast is a Cisco feature for PVST+ environments. When a switch port is configured with PortFast, that port transitions from blocking to forwarding state immediately, bypassing the usual 802.1D STP transition states (the listening and learning states). As shown in Figure 3-40, you can use PortFast on access ports to allow these devices to connect to the network immediately rather than wait for IEEE 802.1D STP to converge on each VLAN. Access ports are ports that are connected to a single workstation or to a server.
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Figure 3-40 PortFast and BPDU Guard Topology
In a valid PortFast configuration, BPDUs should never be received because that would indicate that another bridge or switch is connected to the port, potentially causing a spanning-tree loop. Cisco switches support a feature called BPDU guard. When it is enabled, BPDU guard puts the port in an errdisabled (error-disabled) state on receipt of a BPDU. This effectively shuts down the port. The BPDU guard feature provides a secure response to invalid configurations because you must manually put the interface back into service.
Cisco PortFast technology is useful for DHCP. Without PortFast, a PC can send a DHCP request before the port is in forwarding state, denying the host from getting a usable IP address and other information. Because PortFast immediately changes the state to forwarding, the PC always gets a usable IP address (if the DHCP server has been configured correctly and communication with the DHCP server has occurred).
To configure PortFast on a switch port, enter the spanning-tree portfast interface configuration mode command on each interface on which PortFast is to be enabled, as shown in Example 3-5.
Example 3-5 Configuring PortFast
S2(config)# interface FastEthernet 0/11 S2(config-if)# spanning-tree portfast %Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc... to this interface when portfast is enabled, can cause temporary bridging loops. Use with CAUTION %Portfast has been configured on FastEthernet0/11 but will only have effect when the interface is in a non-trunking mode. S2(config-if)#
The spanning-tree portfast default global configuration mode command enables PortFast on all non-trunking interfaces.
To configure BPDU guard on a Layer 2 access port, use the spanning-tree bpduguard enable interface configuration mode command, as shown in Example 3-6.
Example 3-6 Configuring and Verifying BPDU Guard
S2(config-if)# spanning-tree bpduguard enable S2(config-if)# end S2# S2# show running-config interface f0/11 interface FastEthernet0/11 spanning-tree portfast spanning-tree bpduguard enable S2#
The spanning-tree portfast bpduguard default global configuration command enables BPDU guard on all PortFast-enabled ports.
Notice in Example 3-6 how the show running-config interface command can be used to verify that PortFast and BPDU guard have been enabled for a switch port. PortFast and BPDU guard are disabled, by default, on all interfaces.
PVST+ Load Balancing (3.3.1.4)
The topology in Figure 3-41 shows three switches with 802.1Q trunks connecting them.
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Figure 3-41 PVST+ Configuration Topology
Two VLANs, 10 and 20, are being trunked across these links. The goal is to configure S3 as the root bridge for VLAN 20 and S1 as the root bridge for VLAN 10. Port F0/3 on S2 is the forwarding port for VLAN 20 and the blocking port for VLAN 10. Port F0/2 on S2 is the forwarding port for VLAN 10 and the blocking port for VLAN 20.
In addition to establishing a root bridge, it is also possible to establish a secondary root bridge. A secondary root bridge is a switch that may become the root bridge for a VLAN if the primary root bridge fails. Assuming that the other bridges in the VLAN retain their default STP priority, this switch becomes the root bridge if the primary root bridge fails.
Configuring PVST+ on this topology involves the following steps:
Step 1. Select the switches you want for the primary and secondary root bridges for each VLAN. For example, in Figure 3-41, S3 is the primary bridge for VLAN 20, and S1 is the secondary bridge for VLAN 20.
Step 2. As shown in Example 3-7, configure S3 to be a primary bridge for VLAN 10 and the secondary bridge for VLAN 20 by using the spanning-tree vlan number root { primary | secondary } command.
Example 3-7 Configuring Primary and Secondary Root Bridges for Each VLAN on S3
S3(config)# spanning-tree vlan 20 root primary S3(config)# spanning-tree vlan 10 root secondary
Step 3. As shown in Example 3-8, configure S1 to be a primary bridge for VLAN 20 and the secondary bridge for VLAN 10.
Example 3-8 Configuring Primary and Secondary Root Bridges for Each VLAN on S1
S1(config)# spanning-tree vlan 10 root primary S1(config)# spanning-tree vlan 20 root secondary
Another way to specify the root bridge is to set the spanning-tree priority on each switch to the lowest value so that the switch is selected as the primary bridge for its associated VLAN, as shown in Example 3-9.
Example 3-9 Configuring the Lowest Possible Priority to Ensure That a Switch Is Root
S3(config)# spanning-tree vlan 20 priority 4096
S1(config)# spanning-tree vlan 10 priority 4096
The switch priority can be set for any spanning-tree instance. This setting affects the likelihood that a switch is selected as the root bridge. A lower value increases the probability that the switch is selected. The range is 0 to 61,440, in increments of 4096; all other values are rejected. For example, a valid priority value is 4096 × 2 = 8192.
As shown in Example 3-10, the show spanning-tree active command displays spanning-tree configuration details for the active interfaces only.
The output shown is for S1 configured with PVST+. A number of Cisco IOS command parameters are associated with the show spanning-tree command.
In Example 3-11, the output shows that the priority for VLAN 10 is 4096, the lowest of the three respective VLAN priorities.
Example 3-10 Verifying STP Active Interfaces
S1# show spanning-tree active
<output omitted>
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 4106
Address ec44.7631.3880
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 4106 (priority 4096 sys-id-ext 10)
Address ec44.7631.3880
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/3 Desg FWD 19 128.5 P2p
Fa0/4 Desg FWD 19 128.6 P2p
Example 3-11 Verifying the S1 STP Configuration
S1# show running-config | include span spanning-tree mode pvst spanning-tree extend system-id spanning-tree vlan 1 priority 24576 spanning-tree vlan 10 priority 4096 spanning-tree vlan 20 priority 28672
Packet Tracer 3.3.1.5: Configuring PVST+
In this activity, you will configure VLANs and trunks and examine and configure the Spanning Tree Protocol primary and secondary root bridges. You will also optimize the switched topology by using PVST+, PortFast, and BPDU guard.
Rapid PVST+ Configuration (3.3.2)
Rapid PVST+ is the Cisco implementation of RSTP. It supports RSTP on a per-VLAN basis. The focus of this topic is on how to configure Rapid PVST+ in a switched LAN environment.
Spanning Tree Mode (3.3.2.1)
Rapid PVST+ commands control the configuration of VLAN spanning-tree instances. A spanning-tree instance is created when an interface is assigned to a VLAN, and is removed when the last interface is moved to another VLAN. In addition, you can configure STP switch and port parameters before a spanning-tree instance is created. These parameters are applied when a spanning-tree instance is created.
Use the spanning-tree mode rapid-pvst global configuration mode command to enable Rapid PVST+. Optionally, you can also identify interswitch links as point-to-point links by using the spanning-tree link-type point-to-point interface configuration command. When specifying an interface to configure, valid interfaces include physical ports, VLANs, and port channels.
To reset and reconverge STP, use the clear spanning-tree detected-protocols privileged EXEC mode command.
To illustrate how to configure Rapid PVST+, refer to the topology in Figure 3-42.
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Figure 3-42 Rapid PVST+ Topology
Example 3-12 displays the commands to configure Rapid PVST+ on S1.
Example 3-12 Configuring Rapid PVST+ on S1
S1# configure terminal S1(config)# spanning-tree mode rapid-pvst S1(config)# spanning-tree vlan 1 priority 24576 S1(config)# spanning-tree vlan 10 priority 4096 S1(config)# spanning-tree vlan 20 priority 28672 S1(config)# interface f0/2 S1(config-if)# spanning-tree link-type point-to-point S1(config-if)# end S1# clear spanning-tree detected-protocols
In Example 3-13, the show spanning-tree vlan 10 command shows the spanning-tree configuration for VLAN 10 on switch S1.
Example 3-13 Verifying That VLAN 10 Is Using RSTP
S1# show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 4106
Address ec44.7631.3880
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 4106 (priority 4096 sys-id-ext 10)
Address ec44.7631.3880
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/3 Desg FWD 19 128.5 P2p Peer(STP)
Fa0/4 Desg FWD 19 128.6 P2p Peer(STP)
In the output, the statement “Spanning tree enabled protocol rstp” indicates that S1 is running Rapid PVST+. Notice that the BID priority is set to 4096. Because S1 is the root bridge for VLAN 10, all of its interfaces are designated ports.
In Example 3-14, the show running-config command is used to verify the Rapid PVST+ configuration on S1.
Example 3-14 Verifying the Rapid PVST+ Configuration
S1# show running-config | include span spanning-tree mode rapid-pvst spanning-tree extend system-id spanning-tree vlan 1 priority 24576 spanning-tree vlan 10 priority 4096 spanning-tree vlan 20 priority 28672 spanning-tree link-type point-to-point
Packet Tracer 3.3.2.2: Configuring Rapid PVST+
In this activity, you will configure VLANs and trunks and examine and configure the spanning-tree primary and secondary root bridges. You will also optimize it by using rapid PVST+, PortFast, and BPDU guard.
Lab 3.3.2.3: Configuring Rapid PVST+, PortFast, and BPDU Guard
Refer to Scaling Networks v6 Labs & Study Guide and the online course to complete this activity.
In this lab, you will complete the following objectives:
Part 1: Build the Network and Configure Basic Device Settings
Part 2: Configure VLANs, Native VLAN, and Trunks
Part 3: Configure the Root Bridge and Examine PVST+ Convergence
Part 4: Configure Rapid PVST+, PortFast, BPDU Guard, and Examine Convergence
STP Configuration Issues (3.3.3)
The focus of this topic is on how to analyze common STP configuration issues.
Analyzing the STP Topology (3.3.3.1)
To analyze the STP topology, follow these steps, as shown in the logic diagram in Figure 3-43:
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Figure 3-43 Analyzing the STP Topology
Step 1. Discover the Layer 2 topology. Use network documentation if it exists or use the show cdp neighbors command to discover the Layer 2 topology.
Step 2. After discovering the Layer 2 topology, use STP knowledge to determine the expected Layer 2 path. It is necessary to know which switch is the root bridge.
Step 3. Use the show spanning-tree vlan command to determine which switch is the root bridge.
Step 4. Use the show spanning-tree vlan command on all switches to find out which ports are in blocking or forwarding state and confirm your expected Layer 2 path.
Expected Topology versus Actual Topology (3.3.3.2)
In many networks, the optimal STP topology is determined as part of the network design and then implemented through manipulation of STP priority and cost values, as shown in Figure 3-44.
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Figure 3-44 Verifying That Actual Topology Matches Expected Topology
Situations may occur in which STP was not considered in the network design and implementation, or in which it was considered or implemented before the network underwent significant growth and change. In such situations, it is important to know how to analyze the STP topology in the operational network.
A big part of troubleshooting consists of comparing the actual state of the network against the expected state of the network and spotting the differences to gather clues about the troubleshooting problem. A network professional should be able to examine the switches and determine the actual topology, as well as understand what the underlying spanning-tree topology should be.
Overview of Spanning Tree Status (3.3.3.3)
Using the show spanning-tree command without specifying any additional options provides a quick overview of the status of STP for all VLANs that are defined on a switch.
Use the show spanning-tree vlan vlan_id command to get STP information for a particular VLAN. Use this command to get information about the role and status of each port on the switch. If you are interested only in a particular VLAN, limit the scope of this command by specifying that VLAN as an option, as shown for VLAN 100 in Figure 3-45.
The output on switch S1 in this example shows all three ports in the forwarding (FWD) state and the roles of the three ports as either designated ports or root ports. Any ports being blocked display the output status as “BLK.”
The output also gives information about the BID of the local switch and the root ID, which is the BID of the root bridge.
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Figure 3-45 Overview of STP Status
Spanning Tree Failure Consequences (3.3.3.4)
Figure 3-46 shows a functional STP network. But what happens when there is an STP failure?
There are two types of STP failure. First, STP might erroneously block ports that should have gone into the forwarding state. Connectivity might be lost for traffic that would normally pass through this switch, but the rest of the network remains unaffected. Second, STP might erroneously move one or more ports into the forwarding state, as shown for S4 in Figure 3-47.
Remember that an Ethernet frame header does not include a TTL field, which means that any frame that enters a bridging loop continues to be forwarded by the switches indefinitely. The only exceptions are frames that have their destination address recorded in the MAC address table of the switches. These frames are simply forwarded to the port that is associated with the MAC address and do not enter a loop. However, any frame that is flooded by a switch enters the loop. This may include broadcasts, multicasts, and unicasts with a globally unknown destination MAC address.
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Figure 3-46 STP Switch Topology
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Figure 3-47 Erroneous Transition to Forwarding
Figure 3-48 shows the consequences and corresponding symptoms of STP failure.
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Figure 3-48 Consequences of STP Failure Are Severe
The load on all links in the switched LAN quickly starts increasing as more and more frames enter the loop. This problem is not limited to the links that form the loop but also affects any other links in the switched domain because the frames are flooded on all links. When the spanning-tree failure is limited to a single VLAN only, links in that VLAN are affected. Switches and trunks that do not carry that VLAN operate normally.
If the spanning-tree failure has created a bridging loop, traffic increases exponentially. The switches then flood the broadcasts out multiple ports. This creates copies of the frames every time the switches forward them.
When control plane traffic (for example, routing messages) starts entering the loop, the devices that are running these protocols quickly start getting overloaded. Their CPUs approach 100 percent utilization while they are trying to process an ever-increasing load of control plane traffic. In many cases, the earliest indication of this broadcast storm in progress is that routers or Layer 3 switches report control plane failures and that they are running at a high CPU load.
The switches experience frequent MAC address table changes. If a loop exists, a switch may see a frame with a certain source MAC address coming in on one port and then see another frame with the same source MAC address coming in on a different port a fraction of a second later. This causes the switch to update the MAC address table twice for the same MAC address.
Repairing a Spanning Tree Problem (3.3.3.5)
One way to correct spanning-tree failure is to manually remove redundant links in the switched network, either physically or through configuration, until all loops are eliminated from the topology. When the loops are broken, the traffic and CPU loads should quickly drop to normal levels, and connectivity to devices should be restored.
Although this intervention restores connectivity to the network, it is not the end of the troubleshooting process. All redundancy from the switched network has been removed, and now the redundant links must be restored.
If the underlying cause of the spanning-tree failure has not been fixed, chances are that restoring the redundant links will trigger a new broadcast storm. Before restoring the redundant links, determine and correct the cause of the spanning-tree failure. Carefully monitor the network to ensure that the problem is fixed.
Activity 3.3.3.6: Troubleshoot STP Configuration Issues
Refer to the online course to complete this activity.
Switch Stacking and Chassis Aggregation (3.3.4)
The focus of this topic is to explain the value of switch stacking and chassis aggregation in a small switched LAN.
Switch Stacking Concepts (3.3.4.1)
A switch stack can consist of up to nine Catalyst 3750 switches connected through their StackWise ports. One of the switches controls the operation of the stack and is called the stack master. The stack master and the other switches in the stack are stack members.
Figure 3-49 shows the backplane of four Catalyst 3750 switches and how they are connected in a stack.
Every member is uniquely identified by its own stack member number. All members are eligible masters. If the master becomes unavailable, there is an automatic process to elect a new master from the remaining stack members. One of the factors is the stack member priority value. The switch with the highest stack member priority value becomes the master.
Layer 2 and Layer 3 protocols present the entire switch stack as a single entity to the network. One of the primary benefits of switch stacks is that you manage the stack through a single IP address. The IP address is a system-level setting and is not specific to the master or to any other member. You can manage the stack through the same IP address even if you remove the master or any other member from the stack.
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Figure 3-49 Cisco Catalyst 3750 Switch Stack
The master contains the saved and running configuration files for the stack. Therefore, there is only one configuration file to manage and maintain. The configuration files include the system-level settings for the stack and the interface-level settings for each member. Each member has a current copy of these files for backup purposes.
The switch is managed as a single switch, including passwords, VLANs, and interfaces. Example 3-15 shows the interfaces on a switch stack with four 52-port switches. Notice that the first number after the interface type is the stack member number.
Example 3-15 Switch Stack Interfaces
Switch# show running-config | begin interface interface GigabitEthernet1/0/1 ! interface GigabitEthernet1/0/2 ! interface GigabitEthernet1/0/3 ! <output omitted> ! interface GigabitEthernet1/0/52 ! interface GigabitEthernet2/0/1 ! interface GigabitEthernet2/0/2 ! <output omitted> ! interface GigabitEthernet2/0/52 ! interface GigabitEthernet3/0/1 ! interface GigabitEthernet3/0/2 ! <output omitted> ! interface GigabitEthernet3/0/52 ! interface GigabitEthernet4/0/1 ! interface GigabitEthernet4/0/2 ! <output omitted> ! interface GigabitEthernet4/0/52 ! Switch#
Spanning Tree and Switch Stacks (3.3.4.2)
Another benefit to switch stacking is the ability to add more switches to a single STP instance without increasing the STP diameter. The diameter is the maximum number of switches that data must cross to connect any two switches. The IEEE recommends a maximum diameter of seven switches for the default STP timers. For example, in Figure 3-50, the diameter from S1-4 to S3-4 is nine switches. This design violates the IEEE recommendation.
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Figure 3-50 Diameter Greater Than 7
The recommended diameter is based on default STP timer values, which are as follows:
Hello Timer (2 seconds)—The interval between BPDU updates.
Max Age Timer (20 seconds)—The maximum length of time a switch saves BPDU information.
Forward Delay Timer (15 seconds)—The time spent in the listening and learning states.
Switch stacks help maintain or reduce the impact of diameter on STP reconvergence. In a switch stack, all switches use the same bridge ID for a given spanning-tree instance. This means that, if the switches are stacked, as shown in Figure 3-51, the maximum diameter becomes 3 instead of 9.
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Figure 3-51 Switch Stacking Reduces STP Diameter
Activity 3.3.4.3: Identify Switch Stacking Concepts
Refer to the online course to complete this activity.