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New Member

Minimize the convergence time

I’m about to configure two Catalyst4506 with Sup4 as core switches that should replace the old Catalyst4003. Catalyst3500XL and Catalyst2950G are uses as access-layer switches, they are all running the last IOS release.

I want to minimize the convergence time if one of the switches fails. I have configured Spanning-tree uplinkfast on the access-layer switches. I have read about Backbone fast but this feature are not supported on Catalyst3500XL. Should I use the Backbone fast feature on the c4506 and c2950G? Will Backbone fast be useful since not all access-layer switches support this feature?

Do you have other useful tips that can reduce convergence time on my network?


Re: Minimize the convergence time

You can tweak the Spanning Tree timers from their defaults, to shorten normal STP convergence time of 30 to 50 seconds to something quicker. But don't do this unless you completely understand your network's topology and what you're doing.

Here's a couple of helpful links:

Understanding and Tuning Spanning Tree Protocol Timers

Sample Chapters from the Cisco Press book "Cisco LAN Switching" (the whole Spanning Tree section!)

Understanding Spanning Tree (background knowledge){F4D49AB4-D3EE-4E4C-9077-91AA7A468236}/session_id~{5D563EF7-9D1E-4890-9137-917331A585DF}/catalog/sample_chapter.asp

Advanced Spanning Tree (optimization tips are here){66425BD4-CCB0-468C-8432-A0F7671B440C}/session_id~{5D563EF7-9D1E-4890-9137-917331A585DF}/catalog/sample_chapter.asp

(Sorry for the long URLs. If you can't click it directly, highlight the whole link and copy/paste it to your browser; or just go to the web site and look for the "Cisco LAN Switching" book, you can find the sample chapters there.)

Also, depending on your network design (that is, how your VLANs and Layer3 switching are set up, protocols in use, etc.) maybe tuning STP convergence is not the answer; maybe looking at leveraging HSRP is. But it seems like you're interested in STP.

Also, not sure if all your switches can do Rapid STP (802.1w) or Multiple STP (802.1s). Check into links off the first link I gave you to learn about these; maybe someone else can chime in with exact switch models and software revisions that support these variants of STP.

I have tweaked one customer's STP timers to get convergence time down to about half of what's normal, after reading all of the above; works OK. But almost all the others' networks were better served by the STP defaults, or by taking advantage of HSRP and multigroup HSRP.

Hope all this helps.

Re: Minimize the convergence time


regarding your BackboneFast question:

I've just asked similar question on the "Ask the expert" part of this forum.


My opinion (confirmed by the expert) is that it should be possible to use it if all non-backboneFast switches are access ones. (See "Ask the expert - Spanning Tree Protocol" discussion, there are also some other usefull tips.



New Member

Re: Minimize the convergence time

Which STP are you running ??

MST has a much faster convergence time than PVST+ due to the implementation of RSTP.

Default convergence times drop from 50 secs down to msecs.

New Member

Re: Minimize the convergence time

I have read about MST and if I understood this correctly I should define an MST region which all my Swicthes is member of.

I will also set the first C4506 as STP root primary and the other as STP root secondary. Comments?

Is MST supported on C3500XL?

New Member

Re: Minimize the convergence time


From an admin point of view, it's a good idea to manually set your primary and secondary root rather than leaving it to the STP algorithm.

Sorry I can't help you on the feature set of the C3500XL but that info should be easy to find here at this site.



Re: Minimize the convergence time

New Member

Re: Minimize the convergence time


... And Cisco has announced that WC5 would be the last feature relase for that hardware platform so don't hold your breath waiting for MST on XL.


New Member

Re: Minimize the convergence time

I have near exact the same scenario :

A collapsed core distribution model having 2 Nos Cat 4506 having and access layer switches 5 Nos Cat 3548XL and 3 Nos 2950G .

The Cat 3548 switches are uplinked to both Core 1 and Core 2.

The Cat 2950 switches are connected to each other via gigastack cables and uplinked to the core via OFC.

There are 4 VLANS and no inter VLAN routing.

VLAN1 root primary is Core1 and secondary Core 2

VLAN 2 root primaryis Core 2 and secondary Core 1

Backbone fast was not enabled in any switchas Cat 3548 is notsupporting . Uplinkfast was enabled with 1000 max rate interval in all access switches.

The convergence time was taking 30 secs initialy without any tuning of the spanning tree network dia and hello timers .

When I manually mapped the spanning tree per VLAN I found that at any point of time the network diameter will not be greater than 4.

I am planning to tune the network dia to 4 and hello timer to 1 .so that the convergence time per VLAN primary root and the secondary.

Will that help to reduce the convergence to less than 10secs which is the requirement?



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