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This chapter describes how to configure Spanning-Tree Protocol, and how to use two Spanning-Tree features: Spanning-Tree Uplink Fast Switchover, and Spanning-Tree Protocol Backbone Fast Convergence.
For more information on Spanning-Tree Protocol commands, refer to the Catalyst 5000 Series Command Reference publication.
To enable Spanning-Tree Protocol, enter this command in privileged mode:
| Task | Command |
|---|---|
| Enable Spanning-Tree Protocol. | set spantree enable [vlan] |
After enabling Spanning-Tree Protocol, you see this display:
Console> (enable) set spantree enable 1 VLAN1 bridge spanning tree enabled Console< (enable)
To disable Spanning-Tree Protocol, enter this command in privileged mode:
| Task | Command |
|---|---|
| Disable Spanning-Tree Protocol. | set spantree disable [vlan] |
To set the bridge forward delay for a VLAN, enter this command in privileged mode.
| Task | Command |
|---|---|
| Set the bridge forward delay. | set spantree fwddelay delay [vlan] |
You can configure the type of Spanning-Tree Protocol to be used by a TrBRF. Note that the following Spanning-Tree Protocol and bridge mode configurations are incompatible and can place logical ports in a blocked state:
For more information, see the "Setting the Spanning-Tree Port State" section.
To specify a Spanning-Tree Protocol for a TrBRF, enter this command in privileged mode:
| Task | Command |
|---|---|
| Specify a Spanning-Tree Protocol for a TrBRF. | set vlan vlan_num [stp {ieee | ibm}] |
After entering the set vlan command and specifying a Spanning-Tree Protocol, you see this display:
Console> (enable) set vlan 950 stp ieee Vlan 950 configuration successful Console> (enable) show vlan 950
The set spantree command allows you to use the bridge functional address instead of the IEEE Spanning-Tree Protocol address when a TrBRF is configured to use the IEEE Spanning-Tree Protocol.
To specify that a TrBRF running the IEEE Spanning-Tree Protocol uses the bridge functional address instead of the IEEE Spanning-Tree Protocol address, enter this command in privileged mode:
| Task | Command |
|---|---|
| Specify that a TrBRF running the IEEE Spanning-Tree Protocol use the bridge functional address instead of the IEEE Spanning-Tree Protocol address. | set spantree multicast-address vlan_num ibm |
When you enable Spanning-Tree Protocol, every switch in the network goes through the blocking state and the transitory states at power up. If properly configured, the logical ports then stabilize to the forwarding or blocking state. However, with TrBRFs and TrCRFs, two exceptions require you to manually set the state of a logical port of a TrBRF:
You can use the set spantree portstate command to manually set the state of a logical port to blocked or forwarding mode if either of the above configurations exists.
To manually set the state of a logical port, enter this command in privileged mode:
| Task | Command |
|---|---|
| Manually set the state of a logical port. | set spantree portstate trcrf {auto | block | forward} [trbrf] |
After entering the set spantree portstate command, you see this display:
Console> (enable) set spantree portstate 950 forward Portstate successfully set for tokenring crf 950 Console> (enable)
The port (physical or logical) with the lowest priority value has the highest priority and forwards the spanning-tree frames. The possible priority range is 0 through 255 (decimal). The default is 128. If all ports have the same priority value, the lowest port number forwards the spanning-tree frames.
To configure the priority associated with a logical port, enter this command in privileged mode:
| Task | Command |
|---|---|
| Configure the priority associated with a logical port. | set spantree portpri vlan_num priority |
After entering the set spantree portpri command, you see this display:
Console> (enable) set spantree portpri 900 50 Token Ring 900 port priority set to 50. Console> (enable)
To configure the priority associated with a physical port, enter this command in privileged mode:
| Task | Command |
|---|---|
| Configure the priority associated with a physical port. | set spantree portpri mod_num/port_num priority |
After entering the set spantree portpri command, you see this display:
Console> (enable) set spantree portpri 3/4 10 Bridge port 3/4 port priority set to 10. Console> (enable)
The Spanning-Tree Protocol uses port path costs to determine which port (physical or logical) to select as a forwarding port. You should 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 is 1 to 65535. The default is 62. Path cost is 1000 ÷ LAN speed in megabits per second.
To configure the cost associated with a physical port, enter this command in privileged mode:
| Task | Command |
|---|---|
| Configure the cost associated with a physical port. | set spantree portcost mod_num/port_num cost |
To configure the cost for a logical port, enter this command in privileged mode:
| Task | Command |
|---|---|
| Configure the cost for a logical port. | set spantree portcost trcrf cost |
After entering the set spantree portcost command and specifying a module number and port number, you see this display:
Console> (enable) set spantree portcost 3/4 100 Spantree port 3/4 path cost set to 100. Console> (enable)
You can configure a physical port that is connected to a single workstation or PC to start faster when it is connected, by entering this command in privileged mode:
| Task | Command |
|---|---|
| Configure a physical port that is connected to a single workstation or PC to start faster when it is connected. | set spantree portfast mod_num/port_num {enable | disable} |
You can configure additional Spanning-Tree Protocol parameters using the set spantree command. Table 10-1 lists the command you use.
| Task | Command |
| Set the bridge forward delay for a VLAN. | set spantree fwddelay delay [vlan] |
| Set the bridge hello time for a VLAN | set spantree hello interval |
| Set the bridge maximum aging time for a VLAN. | set spantree maxage agingtime [vlan] |
| Set the bridge priority for a VLAN. | set spantree priority bridge_priority [vlan] |
| Configure a physical port that is connected to a single workstation or PC to start faster when it is connected. | set spantree portfast mod_num/port_num {enable | disable} |
You can display the Spanning-Tree Protocol configuration for a specific VLAN, switching module, or port. To verify the Spanning-Tree Protocol configuration information for a TrBRF or TrCRF, enter this command in privileged mode:
| Task | Command |
|---|---|
| Verify the Spanning-Tree Protocol configuration information. | show spantree vlan_num |
After entering the show spantree command, you see this display:
Console> (enable) show spantree 1003
VLAN 1003
Spanning tree enabled
Designated Root 00-e0-1e-2f-6f-ea
Designated Root Priority 32768
Designated Root Cost 0
Designated Root Port 1/0
Root Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Bridge ID MAC ADDR 00-e0-1e-2f-6f-ea
Bridge ID Priority 32768
Bridge Max Age 20 sec Hello Time 2 sec Forward Delay 15 sec
Port Vlan Port-State Cost Priority Fast-Start Group-method
--------- ---- ------------- ----- -------- ---------- ------------
3/1 1003 not-connected 250 32 disabled
3/2 1003 not-connected 250 32 disabled
3/3 1003 not-connected 250 32 disabled
3/4 1003 not-connected 250 32 disabled
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3/12 1003 not-connected 250 32 disabled
3/13 1003 not-connected 250 32 disabled
3/14 1003 not-connected 250 32 disabled
3/15 1003 not-connected 250 32 disabled
3/16 1003 not-connected 250 32 disabled
Console> (enable)
You can design fault-tolerant connections using Ethernet only or Ethernet combined with other topologies. Figure 10-1 and Figure 10-2 show fault-tolerant Fast Ethernet topologies using the Spanning-Tree Protocol.
Multiple active paths between stations cause loops in the network. If a loop exists in the network, you might receive duplicate messages. When loops occur, some switches see stations on both sides of the switch. This condition confuses the forwarding algorithm and allows duplicate frames to be forwarded.
To provide path redundancy, Spanning-Tree Protocol defines a tree that spans all switches in an extended network. Spanning-Tree Protocol forces certain redundant data paths into a standby (blocked) state. If one network segment in the Spanning-Tree Protocol becomes unreachable, or if Spanning-Tree Protocol costs change, the spanning-tree algorithm reconfigures the spanning-tree topology and reestablishes the link by activating the standby path.
Spanning-Tree Protocol operation is transparent to end stations, which are unaware whether they are connected to a single LAN segment or a switched LAN of multiple segments.
All switches in an extended LAN participating in Spanning-Tree Protocol gather information on other switches in the network through an exchange of data messages called Bridge Protocol Data Units (BPDUs). This exchange of messages results in the following actions:
The Spanning-Tree Protocol root switch is the logical center of the spanning-tree topology in a switched network. All paths that are not needed to reach the root switch from anywhere in the switched network are placed in Spanning-Tree Protocol backup mode. Table 10-2 describes the root switch variables that affect the entire spanning-tree performance.
| Variable | Description |
|---|---|
Hello Time | Determines how often the switch broadcasts its hello message to other switches. |
| Maximum Age Timer | Measures the age of the received protocol information recorded for a port and ensures that this information is discarded when its age limit exceeds the value of the maximum age parameter recorded by the switch. The timeout value for this timer is the maximum age parameter of the switches. |
| Forward Delay Timer | Monitors the time spent by a port in the learning and listening states. The timeout value is the forward delay parameter of the switches. |
BPDUs contain information about the transmitting switch and its ports, including switch and port Media Access Control (MAC) addresses, switch priority, port priority, and port cost. The Spanning-Tree Protocol uses this information 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.
Figure 10-3 shows how BPDUs enable a Spanning-Tree Protocol topology.
The stable active topology of a switched network is determined by the following:
Each configuration BPDU contains the following minimal information:
The switch sends configuration BPDUs to communicate 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 topology changes, instigate a BPDU transmission.
A BPDU exchange results in the following:
If all switches are enabled with default settings, the switch with the lowest MAC address in the network becomes the root switch. The network in Figure 10-4 assumes that Switch A has the lowest MAC address and is therefore the root switch. However, due to traffic patterns, number of forwarding ports, or line types, Switch A might not be the ideal root switch. By increasing the priority (lowering the numerical priority number) of the ideal switch so that it becomes the root switch, you force a Spanning-Tree Protocol recalculation to form a new, stable topology.
When the stable Spanning-Tree Protocol topology is based on default parameters, the path between source and destination stations in a switched network might not be the most 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 point is to make the fastest link the root port.
For example, assume that port 2 on Switch B in Figure 10-4 is a fiber-optic link, and that port 1 on Switch B (a 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 port 2 to a higher priority (lower numerical value) than port 1, port 2 becomes the root port. The same change can occur by changing the Port Cost parameter for port 2 to a lower value than that of port 1.
Each port on a switch using Spanning-Tree Protocol exists in one of the following five states:
A port moves through these five states as follows:
Figure 10-5 illustrates how a port moves through the five states.
You can modify each port state by using management software. When you enable Spanning-Tree Protocol, every switch in the network goes through the blocking state and the transitory states of listening and learning at power up. If properly configured, the ports then stabilize to the forwarding or blocking state.
When the spanning-tree algorithm places a port in the forwarding state, the following occurs:
A port in the blocking state does not participate in frame forwarding, as shown in Figure 10-6. 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.
A port in the blocking state performs as follows:
The listening state is the first transitional state a port enters after the blocking state, when Spanning-Tree Protocol determines that the port should participate in frame forwarding. Learning is disabled in the listening state. Figure 10-7 shows a port in the listening state.
A port in the listening state performs as follows:
A port in the learning state prepares to participate in frame forwarding. The port enters the learning state from the listening state.
A port in the learning state performs as follows:
A port in the forwarding state forwards frames, as shown in Figure 10-8. The port enters the forwarding state from the learning state.
A port in the forwarding state performs as follows:
 Caution
| Use the immediate-forwarding (portfast) mode only on ports connected to individual workstations to allow these ports to come up and go directly to the forwarding state, rather than having to go through the entire spanning-tree initialization process. To prevent illegal topologies, enable Spanning-Tree Protocol on ports connected to switches or other devices that forward messages. |
A port in the disabled state does not participate in frame forwarding or Spanning-Tree Protocol, as shown in Figure 10-9. A port in the disabled state is virtually nonoperational.
A disabled port performs as follows:
Typically, each VLAN runs one instance of Spanning-Tree Protocol to prevent loops in the bridge topology. However, Token Ring runs Spanning-Tree Protocol both at the TrCRF level and the TrBRF level. The Spanning-Tree Protocol that runs at the TrCRF level removes loops in the logical ring. The TrBRF Spanning-Tree Protocol is similar to the Ethernet Spanning-Tree Protocol, interacting with external bridges to remove loops from the bridge topology.
The Catalyst 5000 series Token Ring module supports these Spanning-Tree Protocols:
The Catalyst 5000 series switch uses the IEEE 802.1D and IBM Spanning-Tree Protocols on TrBRFs. The Spanning-Tree Protocol that runs on the TrCRF is either the Cisco or IEEE Spanning-Tree Protocol, depending on the bridging mode you configured for the TrCRF with the set vlan command.
| Caution
| Certain TrBRF Spanning-Tree Protocol and TrCRF bridge mode configurations are incompatible and can place the TrCRFs in a blocked state. For more about these configurations, see the "Setting the Spanning-Tree Port State" section. |
This section describes how to configure the Spanning-Tree Protocol Uplink Fast Switchover feature, which is also called UplinkFast.
UplinkFast provides fast convergence after a spanning-tree topology change and achieves load balancing between redundant links using uplink groups. An uplink group is a set of ports (per VLAN), only one of which is forwarding at any given time. Specifically, an uplink group consists of the root port (which is forwarding) and a set of blocked ports, except for self-looping ports. The uplink group provides an alternate path in case the currently forwarding link fails.
Procedures for configuring UplinkFast appear in this section.
To configure a switch as the primary root switch, enter this command:
| Task | Command |
|---|---|
| Configure a switch as the primary root switch. | set spantree root vlans [dia network_diameter] [hello hello_time] |
After entering the command you see this display. This example shows how to specify a Catalyst 5000 series switch as the primary root switch for VLANs 1-10:
Console> (enable) set spantree root 1-10 dia 4 VLANs 1-10 bridge priority set to 8192 VLANs 1-10 bridge max aging time set to 14 seconds. VLANs 1-10 bridge hello time set to 2 seconds. VLANs 1-10 bridge forward delay set to 9 seconds. Switch is now the root switch for active VLANs 1-6. Console> (enable)
The set spantree root command reduces the bridge priority (the value associated with the switch) from the default (32,768) to a significantly lower value, which allows the switch to become the root switch.
When you specify a switch to become the primary root, the default bridge priority is modified so that it becomes the root for the specified VLANs. Set the bridge priority to 8192. If this setting does not result in the switch becoming a root, modify the bridge priority to be 100 less than the bridge priority of the current root switch. Since different VLANs could potentially have different root switches, the bridge VLAN-priority chosen makes this switch the root for all the VLANs specified. If reducing the bridge priority as low as 1 still does not make the switch the root switch, the system displays a message.
To configure a switch as the secondary root switch, enter this command:
| Task | Command |
|---|---|
| Configure a switch as the secondary root switch. | set spantree root [secondary] vlans [dia network_diameter] [hello hello_time] |
After you enter this command, you see this display. The following example shows how to specify a Catalyst 5000 series switch as the secondary root switch for VLANs 22 and 24:
Console> (enable) set spantree root secondary 22,24 dia 5 hello 1 VLANs 22,24 bridge priority set to 16384. VLANs 22,24 bridge max aging time set to 10 seconds. VLANs 22,24 bridge hello time set to 1 second. VLANs 22,24 bridge forward delay set to 7 seconds. Console> (enable)
The set spantree root secondary command reduces the bridge priority to 16,384, making it the probable candidate to become the root switch if the primary root switch fails. You can run this command on more than one switch to create multiple backup switches in case the primary root switch fails.
To enable and disable UplinkFast, perform these tasks:
| Task | Command |
|---|---|
| Enable the UplinkFast feature. | set spantree uplinkfast enable [rate station_update_rate] [all-protocols off | on] |
| Disable the UplinkFast feature. | set spantree uplinkfast disable |
This command increases the path cost of all ports on the switch, making it unlikely that the switch becomes the root switch. The station_update_rate value represents the number of multicast packets transmitted per 100 milliseconds (the default is 15 packets per millisecond).
After entering the set spantree uplinkfast enable command, you see this display. This example shows how to enable and verify the UplinkFast feature with a station-update rate of 40 packets per 100 milliseconds:
Console> (enable) set spantree uplinkfast enable rate 40 VLANs 1-1000 bridge priority set to 49152. The port cost and portvlancost of all ports increased by 3000. Station update rate set to 40 packets/100ms. uplinkfast turned on for bridge. Console> (enable) Console> (enable) show spantree uplinkfast VLAN port list ------------------------------------------------ 1-20 1/1(fwd),1/2-1/5 21-50 1/9(fwd), 1/6-1/8, 1/10-1/12
To assign a lower cost to a set of VLANs on a port, perform this task:
| Task | Command |
|---|---|
| Lower the cost of a set of VLANs on a port. | set spantree portvlancost mod_num/port_num [cost cost_value] [preferred_vlans] |
If you do not specify the VLANs, the command acts on the VLANs specified in prior instances of this command. If you do not specify a cost, the portvlancost value is set to one less than the current portcost value for the port. This example shows how to assign a lower cost to a set of VLANs on a port:
Console> (enable) set spantree portvlancost 2/10 1-20 Port 2/10 VLANs 1-20 have a path cost of 9.
To achieve load balancing between redundant links, you can modify the path cost of the uplink ports on a per-VLAN basis using the set spantree portvlancost command. To determine the root path cost for a switch through a particular port, the path cost of the port is added to the root path cost defined in the configuration BPDU. You can also achieve load balancing by designating different backbone switches as the root switch for different VLANs.
For descriptions of the commands used to configure the Spanning-Tree Uplink Fast Switchover feature, refer to the Catalyst 5000 Series Command Reference publication.
Switches are normally connected hierarchically, as shown in Figure 10-10. In simpler networks, the upper two levels of the hierarchy may be collapsed into a single backbone layer. Figure 10-10 shows the network topology after spanning tree converges into a loop-free topology. Spanning tree has blocked the redundant links to avoid loops. Every access switch and distribution switch in the figure has a redundant uplink.
Figure 10-11 shows an example topology with no link failures. Switch A, the root switch, is connected directly to Switch B over link L1 and to Switch C over link L2. The port on Switch C that is connected directly to Switch B is in blocking state.
If Switch C detects a link failure on the currently active link L2 (a direct link failure), UplinkFast unblocks the blocked port on Switch C and transitions it to the forwarding state without going through the listening and learning states, as shown in Figure 10-12. This switchover takes approximately one to five seconds.
This section describes how to configure the Spanning-Tree Backbone Fast Convergence feature, also known as the BackboneFast feature.
The BackboneFast Convergence feature reduces the time needed for the spanning tree to converge after experiencing a topology change caused by indirect link failures. This feature complements the UplinkFast feature described in the previous section. However, the Backbone Fast Convergence feature is designed for all switches that experience indirect link failures.
This section contains procedures for configuring BackboneFast Convergence.
To configure the Backbone Fast Convergence feature, enter this command:
| Task | Command |
|---|---|
| Enable the Backbone Fast Convergence feature. | set spantree backbonefast enable |
After enabling Backbone Fast Convergence, you see this display:
Console> (enable) set spantree backbonefast enable Backbonefast enabled for all VLANs.
To display a summary of connected spanning-tree ports by VLAN, enter this command:
| Task | Command |
|---|---|
| Display a summary of connected spanning-tree ports. | show spantree summary |
After entering the show spantree summary command, you see this display:
Console> (enable) show spantree summary
Summary of connected spanning tree ports by vlan
Uplinkfast disabled for bridge.
Backbonefast enabled for bridge.
Vlan Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
1 0 0 0 1 1
Blocking Listening Learning Forwarding STP Active
----- -------- --------- -------- ---------- ----------
Total 0 0 0 1 1
BackboneFast statistics
-----------------------
Number of inferior BPDUs received (all VLANs) : 0
Number of RLQ req PDUs received (all VLANs) : 0
Number of RLQ res PDUs received (all VLANs) : 0
Number of RLQ req PDUs transmitted (all VLANs) : 0
Number of RLQ res PDUs transmitted (all VLANs) : 0
Console> (enable)
To disable the BackboneFast feature, enter this command:
| Task | Command |
|---|---|
| Disable the BackboneFast feature. | set spantree backbonefast disable |
To verify that the BackboneFast feature is enabled, enter this command:
Console> (enable) show spantree backbonefast Backbonefast is enabled.
To verify that the BackboneFast feature is disabled, enter this command:
Console> (enable) show spantree backbonefast Backbonefast is disabled.
The BackboneFast feature is initiated when a root port or blocked port on a switch receives inferior BPDUs from its designated bridge. An inferior BPDU is a BPDU that identifies one switch as both the root bridge and the designated bridge. When a switch receives an inferior BPDU, it indicates that a link to which the switch is not directly connected (an indirect link) has failed (that is, the designated bridge has lost its connection to the root bridge). Under normal spanning-tree rules, the switch ignores inferior BPDUs for the configured maximum aging time, as specified by the agingtime variable of the set spantree maxage command.
The switch tries to determine if it has an alternate path to the root bridge. If the inferior BPDU arrives on a blocked port, the root port and other blocked ports on the switch become alternate paths to the root bridge. (Self-looped ports are not considered alternate paths to the root bridge.) If the inferior BPDU arrives on the root port, all blocked ports become alternate paths to the root bridge. If the inferior BPDU arrives on the root port and there are no blocked ports, the switch assumes that it has lost connectivity to the root bridge, causes the maximum aging time on the root to expire, and becomes the root switch according to normal spanning-tree rules.
If the switch has alternate paths to the root bridge, it uses these alternate paths to transmit a new kind of PDU called the "Root Link Query" PDU. The switch sends the Root Link Query PDU out all alternate paths to the root bridge. If the switch determines that it still has an alternate path to the root, it causes the maximum aging time on the ports on which it received the inferior BPDU to expire. If all the alternate paths to the root bridge indicate that the switch has lost connectivity to the root bridge, the switch causes the maximum aging times on the ports on which it received an inferior BPDU to expire. If one or more alternate paths can still connect to the root bridge, the switch makes all ports on which it received an inferior BPDU its designated ports and moves them out of the blocking state (if they were in blocking state), through the listening and learning states, and into the forwarding state.
Figure 10-13 shows an example topology with no link failures. Switch A, the root switch, connects directly to Switch B over link L1 and to Switch C over link L2. The port on Switch C that connects directly to Switch B is in the blocking state.
If link L1 fails, Switch C detects this failure as an indirect failure, since it is not connected directly to link L1. Switch B no longer has a path to the root switch. The BackboneFast feature allows the blocked port on Switch C to move immediately to the listening state without waiting for the maximum aging time for the port to expire. The BackboneFast feature then transitions the port on Switch C to the forwarding state, providing a path from Switch B to Switch A. This switchover takes approximately 30 seconds. Figure 10-14 shows how the BackboneFast feature reconfigures the topology to account for the failure of link L1.
If a new switch is introduced into a shared-medium topology, the BackboneFast feature is not activated. Figure 10-15 shows a shared-medium topology in which a new switch is added. The new switch begins sending inferior BPDUs that say it is the root switch. However, the other switches ignore these inferior BPDUs and the new switch learns that Switch B is the designated bridge to Switch A, the root switch.
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