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This chapter describes how to configure the spanning-tree PortFast, UplinkFast, and BackboneFast features.
This chapter consists of these sections:
Spanning-tree PortFast causes a spanning-tree port to enter the forwarding state immediately, bypassing the listening and learning states. You can use PortFast on switch ports connected to a single workstation or server to allow those devices to connect to the network immediately, rather than waiting for spanning tree to converge.
 | Caution PortFast should be used only when connecting a single end station to a switch port. Otherwise, you might create a network loop. |
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.
Figure 8-1 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 8-2. This switchover takes approximately one to five seconds.
BackboneFast is initiated when a root port or blocked port on a switch receives inferior BPDUs from its designated bridge. An inferior BPDU 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 8-3 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. BackboneFast 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. BackboneFast 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 8-4 shows how BackboneFast reconfigures the topology to account for the failure of link L1.
If a new switch is introduced into a shared-medium topology, BackboneFast is not activated. Figure 8-5 shows a shared-medium topology in which a new switch is added. The new switch begins sending inferior BPDUs that indicate 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.
These sections describe how to configure spanning-tree PortFast on the switch:
| Caution PortFast should be used only when connecting a single end station to a switch port. Otherwise, you might create a network loop. |
To enable PortFast on a switch port, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Enable PortFast on a switch port connected to a single workstation or server. | set spantree portfast mod_num/port_num enable |
Step 2 Verify the PortFast setting. | show spantree mod_num/port_num |
This example shows how to enable PortFast on a port and verify the configuration (the PortFast status is shown in the Fast-Start column):
Console> (enable) set spantree portfast 1/2 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 1/2 fast start enabled. Console> (enable) show spantree 1/2 Port Vlan Port-State Cost Priority Fast-Start Group-method --------- ---- ------------- ----- -------- ---------- ------------ 1/2 1 blocking 19 20 enabled 1/2 100 forwarding 10 20 enabled 1/2 521 blocking 19 20 enabled 1/2 522 blocking 19 20 enabled 1/2 523 blocking 19 20 enabled 1/2 524 blocking 19 20 enabled 1/2 1003 not-connected 19 20 enabled 1/2 1005 not-connected 19 4 enabled Console> (enable)
To disable PortFast on a switch port, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Disable PortFast on a switch port. | set spantree portfast mod_num/port_num disable |
Step 2 Verify the PortFast setting. | show spantree mod_num/port_num |
This example shows how to disable PortFast on a port and verify the configuration (the PortFast status is shown in the Fast-Start column):
Console> (enable) set spantree portfast 1/2 disable Spantree port 1/2 fast start disabled. Console> (enable) show spantree 1/2 Port Vlan Port-State Cost Priority Fast-Start Group-method --------- ---- ------------- ----- -------- ---------- ------------ 1/2 1 blocking 19 20 disabled 1/2 100 forwarding 10 20 disabled 1/2 521 blocking 19 20 disabled 1/2 522 blocking 19 20 disabled 1/2 523 blocking 19 20 disabled 1/2 524 blocking 19 20 disabled 1/2 1003 not-connected 19 20 disabled 1/2 1005 not-connected 19 4 disabled Console> (enable)
These sections describe how to configure UplinkFast on the switch:
The set spantree uplinkfast enable command increases the path cost of all ports on the switch, making it unlikely that the switch will become 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).
To enable UplinkFast, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Enable UplinkFast on the switch. | set spantree uplinkfast enable [rate station_update_rate] [all-protocols off | on] |
Step 2 Verify that UplinkFast is enabled. | show spantree uplinkfast |
This example shows how to enable UplinkFast with a station-update rate of 40 packets per 100 milliseconds and how to verify that UplinkFast is enabled:
Console> (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. Console> (enable) show spantree uplinkfast Station update rate set to 15 packets/100ms. uplinkfast all-protocols field set to off. VLAN port list ----------------------------------------------- 1 1/1(fwd),1/2 100 1/2(fwd) 521 1/1(fwd),1/2 522 1/1(fwd),1/2 523 1/1(fwd),1/2 524 1/1(fwd),1/2 Console> (enable)
The set spantree uplinkfast enable command increases the path cost of all ports on the switch, making it unlikely that the switch will become 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).
To enable UplinkFast, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Disable UplinkFast on the switch. | set spantree uplinkfast disable |
Step 2 Verify that UplinkFast is enabled. | show spantree uplinkfast |
This example shows how to disable UplinkFast on the switch and verify the configuration:
Console> (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. Console> (enable) show spantree uplinkfast Station update rate set to 15 packets/100ms. uplinkfast all-protocols field set to off. VLAN port list ----------------------------------------------- 1 1/1(fwd),1/2 100 1/2(fwd) 521 1/1(fwd),1/2 522 1/1(fwd),1/2 523 1/1(fwd),1/2 524 1/1(fwd),1/2 Console> (enable)
These sections describe how to configure BackboneFast:
To enable BackboneFast, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Enable BackboneFast on the switch. | set spantree backbonefast enable |
Step 2 Verify that BackboneFast is enabled. | show spantree backbonefast |
This example shows how to enable BackboneFast on the switch and how to verify the configuration:
Console> (enable) set spantree backbonefast enable Backbonefast enabled for all VLANs Console> (enable) show spantree backbonefast Backbonefast is enabled. Console> (enable)
To display BackboneFast statistics, perform this task in privileged mode:
| Task | Command |
|---|---|
Display BackboneFast statistics. | show spantree summary |
This example shows how to display BackboneFast statistics:
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 BackboneFast, perform this task in privileged mode:
| Task | Command |
|---|---|
Step 1 Disable BackboneFast on the switch. | set spantree backbonefast disable |
Step 2 Verify that BackboneFast is disabled. | show spantree backbonefast |
This example shows how to disable BackboneFast on the switch and how to verify the configuration:
Console> (enable) set spantree backbonefast disable Backbonefast enabled for all VLANs Console> (enable) show spantree backbonefast Backbonefast is disable. Console> (enable)
Posted: Mon May 1 12:14:19 PDT 2000
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