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Basic STP Configuration
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STP configuration for a given switch varies depending on the environment and switch type (access, distribution, or core) in question You first need to determine whether STP is even necessary In most small environments, redundant paths between switches may not even exist In these cases, bandwidth can be spared and performance slightly improved by disabling STP entirely In a larger or more redundant environment, you will need to leave STP enabled and ask yourself a few questions concerning the network layout to determine the proper base STP configuration, such as the following:
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Are any non-Cisco switches present What is the network diameter (number of switches through which any frame must pass) Which switch would make the best STP root Should Backbonefast be enabled Which switches should have Uplinkfast enabled Which access links should have Portfast enabled
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First, let's look at how to disable STP entirely In a smaller environment, disabling STP can provide a small performance improvement, but you need to make absolutely sure that no loops are present in the topology (The topology must be a true physical star) To disable spanning tree on a standard IOS-based switch (2948G and 3500 series), issue the no spanning-tree [vlan list] command On the 1900 series, issue the no spantree [vlan list] command; and on a CatOS-based switch, issue the set spantree disable all command Spanning tree should be enabled by default, but if you need to re-enable it, simply issue the opposite command (spanning-tree [vlan list], spantree [vlan list], or set spantree enable all) Note On some versions of the CatOS supervisor engines, a VLAN list is supported after the set spantree enable or set spantree disable command The examples used in this chapter are from a Catalyst 5500 with the Supervisor IIIG engine, which supports only enabling or disabling of spanning tree for all VLANs (rather than for specific VLANs) The following output shows examples of these commands for all the switches mentioned:
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>1900 Switch Configuration< 1924(config)# no spantree vlan 1 2 3 4 1924(config)# spantree vlan 1 2 3 4 >3500 Switch Configuration< 3508(config)# no spanning-tree vlan 200 3508(config)# spanning-tree vlan 200 >5500 Switch Configuration<
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Cat5K (enable) set spantree disable all Spantree disabled Cat5K (enable) set spantree enable all Spantree enabled
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After you choose to enable spanning tree, you must determine the proper STP configuration First, determine whether any non-Cisco switches are present in the environment This information lets you know which Cisco proprietary enhancements you can use, such as Fast Etherchannel and Per VLAN Spanning Tree (PVST) Per VLAN Spanning Tree is discussed in the section "VLAN STP Considerations," later in this chapter Next, determine the switch diameter By setting the switch diameter, you cause STP to recalculate timer values to help ensure optimal performance This technique is the primary, and typically the best, way of setting the timer values Manual modification of the timers is not suggested, because improper settings can reduce performance or cause looping To calculate the diameter, examine the number of switches between any two hosts in the network The default STP diameter is seven, which is also the maximum setting In most networks, this number can be reduced to four or less to improve performance (Remember, the Ethernet specification allows only four repeaters, including switches, between any two hosts) To configure the STP diameter on a standard IOS switch, use the spanning-tree [vlan list] root primary diameter [value] command Note that you can configure the diameter only on the STP root, because it is the switch that controls the spanning tree timers The root primary parameter in this command sets the switch's priority to 8192, almost ensuring that it will become the STP root On a CatOS-based switch, use the command set spantree root [vlan list] dia [diameter value] As part of this process, you will by necessity be forced to choose the STP root The STP root bridge should generally be a fast switch near or at the center of your network topology Remember, the majority of your network traffic will likely flow through the root bridge, so you do not want the root bridge to be an underpowered or poorly positioned switch Figure 19-12 shows a good example of a root bridge selection and the resulting traffic paths You can choose the root bridge by either using the commands just outlined (spanning-tree [vlan list] root or set spantree root), or by setting the priority manually by using the spanning-tree priority [value] or set spantree priority [value] commands
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Figure 19-12: Proper root selection You should also choose a secondary root bridge in case the primary root bridge fails You can use the spanning-tree [vlan list] root secondary or set spantree root secondary command to make secondary root bridge selection happen automatically (setting the priority to 16,384) You can, of course, manually set the priority as well by using the spanning-tree priority [value] or set spantree priority [value] commands and entering a value that is less than the default (32,768) but higher than the primary root's priority An example of setting a switch to be the primary root, along with setting the priority manually, is provided for both a standard IOS and CatOS-based switch:
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>3500 Switch Configuration< 3508(config)# spanning-tree vlan 1 root primary diameter 7 3508(config)# spanning-tree vlan 1 priority 100 >5500 Switch Configuration< Cat5K (enable) set spantree root 1 dia 7 VLAN 1 bridge priority set to 8192 VLAN 1 bridge max aging time set to 20 VLAN 1 bridge hello time set to 2 VLAN 1 bridge forward delay set to 15 Switch is now the root switch for active VLAN 1 Cat5K (enable) set spantree root secondary 1 dia 7 VLAN 1 bridge priority set to 16384 VLAN 1 bridge max aging time set to 20 VLAN 1 bridge hello time set to 2 VLAN 1 bridge forward delay set to 15 Cat5K (enable) set spantree priority 100 Spantree 1 bridge priority set to 100
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In addition, in a large network with multiple administrative entities, you may wish to enable a special Cisco switching feature, root guard Because in a large environment, you can never be sure of what other administrators will configure, it is best in some cases to guard against unwanted root selection when another administrator configures a better priority on an inferior
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switch Root guard keeps a rogue switch from becoming the STP root and forcing an STP topology change by disabling ports that hear of a better root than the current root For instance, Figure 19-13 shows a proper STP configuration
Figure 19-13: Base topology before adding the new switch Then, an administrator adds a new switch and sets the spanning tree priority for the switch to 0 Under normal situations, this setting would force the topology change shown in Figure 1914
Figure 19-14: Incorrect topology after adding the new switch However, if you had enabled root guard on the ports the administrator was connecting to, as well as all ports along the new switch's path back to the correct root, root guard would have
blocked these ports, disabling data traffic to the new switch until such time as the administrator set the priority back to something reasonable Figure 19-15 shows this situation
Figure 19-15: Where root guard needs to be enabled to protect the topology To configure root guard on a standard IOS switch, use the spanning-tree guard root command in interface config mode for the specific interface that is required On a CatOS-based switch, use the set spantree guard root [module/port] command These commands are shown in the following output:
>3500 Switch Configuration< 3508(config)#interface GigabitEthernet 0/8 3508(config-if)#spanning-tree rootguard >5500 Switch Configuration< Cat5K> (enable) set spantree guard root 3/12 Rootguard on port 3/12 is enabled Warning!! Enabling rootguard may result in a topology change Cat5K> (enable) set spantree guard none 3/12 Rootguard on port 3/12 is disabled
Note Be very careful about where you enable root guard If you enable it on all ports or improper ports, you might lose data and have inconsistent STP configurations Choosing which switches to enable Backbonefast on is a bit easier Typically, all backbone ports on all switches can benefit from Backbonefast To enable Backbonefast on a standard IOS-based switch, use the spanning-tree backbonefast command On a CatOS-based switch, use the set spantree backbonefast command The only caveat in using Backbonefast is that for it to function, Backbonefast must be enabled on all switches in the topology For Uplinkfast, the decision is nearly as simple as Backbonefast Uplinkfast should typically be enabled only on access-layer switches, as shown in Figure 19-16 Enabling Uplinkfast on core or distribution switches can cause the STP topology to become inconsistent and hamper performance However, on access-layer switches, Uplinkfast can reduce the time to reconfigure the STP topology after a direct link failure from 30 seconds to less than 5 seconds, which can help eliminate buffer overflows and lost data due to STP delays after link failures
Figure 19-16: Correct switch choices for Uplinkfast To enable Uplinkfast on a standard IOS switch, simply enter the command spanning-tree uplinkfast in global config mode For CatOS-based switches, use the set spantree uplinkfast enable command Finally, Portfast configuration is a bit more complicated than Uplinkfast and Backbonefast configuration First, unlike the other two enhancements, Portfast is port specific, requiring that you manually configure it on a per-port basis Also, using Portfast on links to other switches can cause serious STP problems, so you must be careful when deciding on the ports to enable for Portfast However, when used correctly, Portfast can transition a port into the forwarding state almost immediately upon connection This quick response allows hosts that require immediate network access upon boot to receive that access with little or no drama Basic Portfast selection and configuration is fairly simple First, determine which links are access links for network hosts For instance, in the example in Figure 19-17, Ports 1 and 3 on Corvette and Ports 4 and 5 on Viper should have Portfast enabled Once the proper ports have been selected, use the spanning-tree portfast interface config command (on standard IOS switches) or the set spantree portfast [module/port] enable command (on CatOS switches)
Figure 19-17: Proper Portfast selection Next, you may want to enable an additional Cisco feature for Portfast, called BPDU guard BPDU guard disables any Portfast-configured port if a BPDU is received on that port It performs this action because, due to the removal of the waiting period for a port to transition to a forwarding state, loops can be caused if a Portfast port is connected to another switch If a port is connected to another switch, spanning tree BPDUs should begin entering the port BPDU guard listens for these BPDUs, and if it detects one, it will immediately disable the port to prevent a loop To enable BPDU guard on a standard IOS switch, use the spanningtree portfast bpdu-guard global config command On a CatOS switch, use the set spantree portfast bpdu-guard command Basic Backbonefast, Uplinkfast, and Portfast configuration commands for both Catalyst 5500 and 3500 switches are shown in the following output:
>3500 Switch Configuration< 3508(config)# spanning-tree backbonefast 3508(config)# spanning-tree uplinkfast 3508(config)#interface GigabitEthernet 0/8 3508(config-if)# spanning-tree portfast >5500 Switch Configuration< Cat5K (enable) set spantree uplinkfast enable VLANs 1-1005 bridge priority set to 49152 The port cost and portvlancost of all ports set to above 3000 Station update rate set to 15 packets/100ms uplinkfast all-protocols field set to off uplinkfast enabled for bridge Cat5K (enable) set spantree uplinkfast disable uplinkfast disabled for bridge Use clear spantree uplinkfast to return stp parameters to default Cat5K (enable) clear spantree uplinkfast This command will cause all portcosts, portvlancosts, and the bridge priority on all vlans to be set to default Do you want to continue (y/n) [n] y VLANs 1-1005 bridge priority set to 32768 The port cost of all bridge ports set to default value The portvlancost of all bridge ports set to default value uplinkfast all-protocols field set to off uplinkfast disabled for bridge Cat5K (enable) set spantree portfast 3/1 enable Warning: Spantree port fast start should only be enabled on ports connected
to a single host Connecting hubs, concentrators, switches, bridges, etc to a fast start port can cause temporary spanning tree loops Use with caution Spantree port 3/1 fast start enabled Cat5K (enable) set spantree portfast 3/1 disable Spantree port 3/1 fast start disabled Cat5K (enable) set spantree backbonefast enable Backbonefast enabled for all VLANs Cat5K (enable) set spantree backbonefast disable Backbonefast disabled for all VLANs
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