7-3. STP Convergence Tuning STP bases its operation on several timers. Usually, the default timer values are used for proper STP behavior. The defaults are based on a network diameter of seven switches but can be adjusted for faster convergence times. - The Hello Timer triggers periodic hello messages to neighboring switches. - The Forward Delay timer specifies the time a port stays in each of the listening and learning states. - The MaxAge timer specifies the lifetime of a stored BPDU received on a designated port. After the timer expires, other ports can become designated ports.
BPDUs are expected at regular intervals. If they are delayed beyond the lapse of an STP timer, topology changes can be triggered in error. This condition can be detected with the BPDU skewing feature. STP PortFast allows ports that connect to hosts or nonbridging network devices to enter the forwarding mode immediately when the link is established. This bypasses the normal STP port states for faster startup, but allows the potential for bridging loops to form. STP UplinkFast is used only on leaf-node switches (the ends of the ST branches), usually located in the access layer. The switch keeps track of all potential paths to the root, which are in the blocking state. - When the root port fails, an alternate port is brought into the forwarding state without the normal STP port state progression and delays. - When UplinkFast is enabled, the bridge priority is raised to 49152, making it unlikely to become the root bridge. All switch ports have their port costs increased by 3000 so that they won't be chosen as root ports. - When an alternate root port comes up, the switch updates upstream switches with the new location of downstream devices. Dummy multicasts are sent to destination 01-00-0C-CD-CD-CD that contains the MAC addresses of stations in the bridging table.
STP BackboneFast causes switches in the network core to actively look for alternate paths to the root bridge in case of an indirect failure. - When used, this feature should be enabled on all switches in the network. Switches use a request-and-reply mechanism to determine root path stability, so all switches must be able to participate. - BackboneFast can only reduce the convergence delay from the default 50 seconds (20 seconds for the MaxAge timer to expire, and 15 seconds in both listening and learning states) to 30 seconds.
Configuring STP Convergence Tuning 1. | (Optional) Tune the STP timers to adjust convergence.
NOTE STP timer values should only be modified on the root bridge. The root will propagate the values to all other switches through its configuration BPDUs. If you think that the STP timers must be adjusted, consider doing this by setting the network diameter on the STP root bridge. After the diameter has been set, all the other STP timer values are computed and adjusted automatically. Refer to Step 4 in section "7-2: STP Configuration" for more details. - a. (Optional) Adjust the STP Hello timer:
COS | [View full width] PVST+: set spantree hello interval [vlan-list] MISTP: set spantree hello interval mistp-instance  instances MST: set spantree hello interval mst
| IOS | [View full width] PVST+: (global) spanning-tree [vlan vlan] hello-time interval
|
The Hello timer can be set to interval (1 to 10 seconds, default 2 seconds). It can be set for specific VLANs or STP instances, or globally for VLAN 1 (COS) or all VLANs (IOS) if the VLAN number is not given.
- b. (Optional) Adjust the STP Forward Delay timer:
COS | [View full width] PVST+: set spantree fwddelay delay [vlans] MISTP: set spantree fwddelay delay mistp-instance [instances] MST: set spantree fwddelay delay mst
| IOS | PVST+: spanning-tree vlan vlan forward-time delay
|
The Forward Delay interval can be set to delay (4 to 30 seconds, default 15 seconds) for specific VLANs, specific instances, or globally for VLAN 1 (COS) or all VLANs (IOS) if the VLAN number is not given.
- c. (Optional) Adjust the STP MaxAge timer:
COS | [View full width] PVST+: set spantree maxage agingtime [vlan] MISTP: set spantree maxage agingtime mistp-instance instances MST: set spantree maxage agingtime mst
| IOS | [View full width] PVST+: (global) spanning-tree [vlan vlan] max-age agingtime
|
The MaxAge timer can be set to agingtime (6 to 40 seconds, default 20 seconds) for specific VLANs, instances, or globally for VLAN 1 (COS) or all VLANs (IOS) if the VLAN number is not given.
| 2. | (Optional) Detect BPDU skewing problems:
COS | set spantree bpdu-skewing {enable | disable}
| IOS | N/A |
For troubleshooting purposes, you can detect the delay or "skew" in BPDU reception by using the enable keyword (disabled by default). Any BPDU skewing events are logged on the switch. You can then use the show spantree bpdu-skewing vlan [mod/port] command to view the skew statistics that have been gathered.
| 3. | (Optional) Use PortFast STP convergence for access layer nodes.
- a. Use PortFast on specific ports:
COS | [View full width] set spantree portfast mod/port {enable [trunk] | disable | default}
| IOS | (interface) spanning-tree portfast
|
You can enable or disable PortFast on nontrunking ports. As well, you can use the keyword trunk to force PortFast to be used on a trunking link. The default keyword causes the port to be returned to its default behavior, configured by the optional command set spantree global-default portfast {enable | disable}.
NOTE Enabling PortFast on a port also prevents TCN BPDUs from being generated due to a state change on the port. Although STP is still operating on the port to prevent bridging loops, topology changes are not triggered when the attached host goes up or down. You should use the PortFast feature only on switch ports where single hosts are connected. In other words, don't enable PortFast on switch ports that connect to other switches or hubs, whether the ports or trunking or not. - b. (Optional) Enable PortFast BPDU Guard to improve STP stability:
COS | [View full width] set spantree portfast bpdu-guard mod/port {enable | disable | default}
| IOS | (global) spanning-tree portfast bpduguard
|
On a nontrunked port with PortFast enabled, BPDU Guard moves the port into the Errdisable state if a BPDU is detected. You can use enable or disable to control the BPDU filtering state on the port. Use the default keyword to return the port to the global default set by the optional command set spantree global-default bpdu-guard {enable | disable}.
On the Supervisor IOS, BPDU Guard is enabled globally on all ports that have PortFast enabled.
NOTE This Supervisor IOS command is not available on the Catalyst 2900XL and 3500XL family switches. - c. (Optional) Enable PortFast BPDU Filtering to stop BPDU processing on a port:
COS | [View full width] set spantree portfast bpdu-filter mod/port {enable | disable | default}
| IOS | N/A |
BPDU Filtering causes the switch to stop sending BPDUs on the specified port. As well, incoming BPDUs on that port will not be processed. You can use enable or disable to control the BPDU Filtering state on the port. Use the default keyword to return the port to the global default set by the optional command set spantree global-default bpdu-filter {enable | disable}.
| 4. | (Optional) Use UplinkFast STP convergence for access layer uplinks:
COS | [View full width] set spantree uplinkfast {enable | disable} [rate station_update_rate] [all-protocols {off | on}]
| IOS | [View full width] (global) spanning-tree uplinkfast [max-update-rate packets-per-second]
|
The switch can generate dummy multicasts at a rate up to station-update-rate per 100 milliseconds (Catalyst OS, default 15 per 100 ms) or packets-per-second (Supervisor IOS, default 150 packets per second). The all-protocols keyword can be used to generate multicasts for each protocol filtering group.
TIP To disable UplinkFast and return the bridge priority and port costs to their original values, use the clear spantree uplinkfast COS command. TIP If you are using GigaStack stacking GBIC modules on a Catalyst 2900XL, 3500XL, or 3550, you can connect more than one switch to a single Gigabit Interface Converter (GBIC) port. The stacking GBIC, having two physical connections, becomes a multidrop backbone, making the normal UplinkFast feature ineffective during a topology change. You can enable Cross-Stack UplinkFast (CSUF) in addition to UplinkFast, allowing fast failover through the stacking GBIC connection. You can enable CSUF on only one stacking GBIC interface per switch and should enable it on all switches connected to the stack: COS | N/A | IOS | (interface) spanning-tree stack-port
|
| 5. | (Optional) Use BackboneFast STP convergence for redundant backbone links:
COS | set spantree backbonefast {enable | disable}
| IOS | (global) spanning-tree backbonefast
|
When used, you should enable on all switches in the network. BackboneFast is enabled or disabled for all VLANs on the switch.
|
|
