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Chapter 7: Spanning Tree Protocol (STP) 125

Section 7-3

■ 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 be modified only on the root bridge. The root propagates

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 net-

work 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:

(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:

(global)spanning-tree vlan vlan forward-time delay

The Forward Delay interval can be set to delay (4 to 30 seconds, default 15 sec-

onds) 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:

(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) Use PortFast STP convergence for access layer nodes.

a. Use PortFast on specific ports:

(interface) spanning-tree portfast [trunk]

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.

Cisco Catalyst 2000 User Manual

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