|
|
This chapter describes how to configure the spanning tree PortFast, UplinkFast, and BackboneFast features on the Catalyst 6000 family switches.
This chapter consists of these sections:
Spanning tree PortFast causes an interface configured as a Layer 2 access port to enter the forwarding state immediately, bypassing the listening and learning states. You can use PortFast on Layer 2 access 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. If the interface receives a Bridge Protocol Data Unit (BPDU), which should not happen if the interface is connected to a single workstation or server, spanning tree puts the port into the blocking state.
 | Caution Use PortFast only when connecting a single end station to a Layer 2 access port. Otherwise, you might create a network loop. |
When the BPDU guard feature is enabled on the switch, spanning tree shuts down PortFast-configured interfaces that receive BPDUs, rather than putting them into the spanning tree blocking state. In a valid configuration, PortFast-configured interfaces do not receive BPDUs. Reception of a BPDU by a PortFast-configured interface signals an invalid configuration, such as connection of an unauthorized device. The BPDU guard feature provides a secure response to invalid configurations, because the administrator must manually put the interface back in service.
UplinkFast provides fast convergence after a direct link failure and achieves load balancing between redundant Layer 2 links using uplink groups. An uplink group is a set of Layer 2 interfaces (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 11-1 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 Layer 2 interface 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 on the root port (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 11-2. 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 spanning tree max-age 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 Protocol Data Unit (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 the blocking state), through the listening and learning states, and into the forwarding state.
Figure 11-3 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 Layer 2 interface on Switch C that connects directly to Switch B is in the blocking state.
If link L1 fails, Switch C cannot detect this failure since it is not connected directly to link L1. However, because Switch B is directly connected to the root switch over L1, it detects the failure and elects itself the root and begins sending BPDUs to Switch C indicating itself as the root. When Switch C receives the inferior BPDUs from Switch B, Switch C infers that an indirect failure has occured. At that point 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 Layer 2 interface on Switch C to the forwarding state, providing a path from Switch B to Switch A. This switchover takes approximately 30 seconds, twice the Forward Delay time if the default Forward Delay time of 15 seconds is set. Figure 11-4 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 as shown in Figure 11-5, BackboneFast is not activated because the inferior BPDUs did not come from the recognized designated bridge (Switch B). 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.
| Caution Use PortFast only when connecting a single end station to a Layer 2 access port. Otherwise, you might create a network loop. |
To enable PortFast on a Layer 2 access port, perform this task:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config)# interface {{ethernet | fastethernet | gigabitethernet} slot/port} | {port-channel port_channel_number}
| Select an interface to configure. | ||
| Router(config-if)# spanning-tree portfast | Enable PortFast on a Layer 2 access port connected to a single workstation or server. | ||
| Router(config-if)# exit | Exit interface configuration mode. | ||
| Router(config)# exit | Exit configuration mode. | ||
| Router# | Verify the configuration. |
This example shows how to enable PortFast on Fast Ethernet interface 5/8:
Router# configure terminal Router(config)# interface fastethernet 5/8 Router(config-if)# spanning-tree portfast Router(config-if)# exit Router(config)# exit Router#
This example shows how to verify the configuration:
Router# show running-config interface fastEthernet 5/8 Building configuration... Current configuration: ! interface FastEthernet5/8 no ip address switchport switchport access vlan 200 switchport mode access spanning-tree portfast end Router#
To enable the BPDU guard feature on the switch, perform this task:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config)# spanning-tree portfast bpdu-guard | Enable BPDU guard on the switch. | ||
| Router(config)# exit | Exit configuration mode. | ||
| Router# | Verify the configuration. |
This example shows how to enable BPDU guard on the switch:
Router# configure terminal Router(config)# spanning-tree portfast bpdu-guard Router(config)# exit Router#
This example shows how to verify the configuration:
Router# show spanning-tree summary totals
UplinkFast is disabled
BackboneFast is disabled
PortFast BPDU Guard is enabled
Name Blocking Listening Learning Forwarding STP Active
-------------------- -------- --------- -------- ---------- ----------
34 VLANs 0 0 0 36 36
Router#
Uplinkfast increases the bridge priority to 49152 and adds 3000 to the spanning tree port cost of all interfaces on the switch, making it unlikely that the switch will become the root switch. The max_update_rate value represents the number of multicast packets transmitted per second (the default is 150 packets per second). UplinkFast cannot be enabled on VLANs that have been configured for bridge priority. To enable UplinkFast on a VLAN with bridge priority configured, restore the bridge priority on the VLAN to the default value by issuing a no spanning-tree vlan vlan-id priority command in global configuration mode.
To enable UplinkFast, perform this task:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config)# spanning-tree uplinkfast [max-update-rate max_update_rate] | Enable UplinkFast on the switch. | ||
| Router(config)# exit | Exit configuration mode. | ||
| Router# | Verify that UplinkFast is enabled. |
This example shows how to enable UplinkFast with an update rate of 400 packets per second:
Router# configure terminal Router(config)# spanning-tree uplinkfast max-update-rate 400 Router(config)# exit Router#
This example shows how to verify that UplinkFast is enabled:
Router# show spanning-tree uplinkfast UplinkFast is enabled Router#
To enable BackboneFast, perform this task:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| Router(config)# spanning-tree backbonefast | Enable BackboneFast on the switch. | ||
| Router(config)# exit | Exit configuration mode. | ||
| Router# | Verify that UplinkFast is enabled. |
This example shows how to enable BackboneFast on the switch:
Router# configure terminal Router(config)# spanning-tree backbonefast Router(config)# exit Router#
This example shows how to verify that BackboneFast is enabled:
Router# show spanning-tree backbonefast BackboneFast is enabled BackboneFast statistics ----------------------- Number of transition via backboneFast (all VLANs) : 0 Number of inferior BPDUs received (all VLANs) : 0 Number of RLQ request PDUs received (all VLANs) : 0 Number of RLQ response PDUs received (all VLANs) : 0 Number of RLQ request PDUs sent (all VLANs) : 0 Number of RLQ response PDUs sent (all VLANs) : 0 Router#
Posted: Mon Jan 3 14:37:19 PST 2000
Copyright 1989-1999©Cisco Systems Inc.