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New Member

Spanning Tree Configuration Clarification


We are implementing STP between 3 switches. Two, 100mb ports will be configured into an EtherChannel and Trunking setup to handle the VLANS between the 3 switches. To improve convergence of the network in the event of an uplink failure, BackboneFast - BPDU Guard and UDLD will be configured on all three switches. UplinkFast will be configured on two of the three switches.

First question: Is my "Design" correct in regards to configuring UplinkFast on only 2 of the 3 switches.

Next, UDLD has an assumed dependency upon STP's default timers.

Second question: Does UplinkFast and BackboneFast tune/change the default timers of STP? IE: Should I be considering CatOS 6.2 and the Loopguard features of this IOS?


Cisco Employee

Re: Spanning Tree Configuration Clarification

Configure your etherchannel and trunking as desirable, that will give you some failback configuration in the event of a failure. Use UDLD aggressive-mode instead of standard UDLD as it has more aggressive timers that can prevent STP loops.

Uplinkfast is only design for having 1 switch failover to a second core switch. Since only 1 of 3 switches will have a blocking port in this design, Uplinkfast only has meaning on 1 of the switches.

Uplinkfast and Backbonefast do not change the default timers, but bypass some of the states of not forwarding packets.

I would consider 6.3.x over 6.2.x since it has more bug fixes.

Last, have you look at using Rapid STP 802.1w/802.1s

New Member

Re: Spanning Tree Configuration Clarification

Thank You!

As I understand this, the "Access" switch with the blocked port, should have UplinkFast configured. This is a design consideration, considering the other "Access" switch does not have a "blocking" port during normal operations. Should the uplink between the two access switches fail, Uplinkfast will put the blocked port into forwarding. Putting it back to blocking mode, only after the failed link is stable, and all switches have calculated the CBPDU's.

Configuring Uplinkfast on both access switches would override this, and cause major STP problems, if the uplink between the 2 access switches is flapping.

That makes sense!

The switches are Cat5000's, and I've not found any configuration information regarding Rapid STP for the Cat5000.

However, we are implementing a Cat4000, sometime soon. What would be your recomendation on IOS versions for the C5000's in the environment, when mixed with the C4000? Are there major compelling reasons to upgrade to IOS 6.3?

Thank you again!

Cisco Employee

Re: Spanning Tree Configuration Clarification

Rapid STP requires 7.x software which is not supported on Cat5k. Cat5k only need 5.5.x software, I see no compeling reason to run 6.3.x on Cat5k other than a few features.

For the Cat4k, I would definitely consider 6.3.x. 6.x added numerous troubleshooting enhancements and features. These troubleshooting enhancements aid in the detection of bad hardware, etc.

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