Which spanning tree to use?

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Oct 5th, 2007

Why use STP if RSTP is so much faster? In what instances would it be a bad idea to use rstp?


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Francois Tallet Fri, 10/05/2007 - 12:52

I would definitely encourage you to run RSTP (if not MST) instead of STP. RSTP will interact gracefully with STP anyway.

In order to benefit from RSTP convergence speed, the following condition must be met:

-1- the inter-switch links have to be point to point (i.e. links with only two bridges on them). This is a rather straightforward requirement.

-2- all the switches must be configured to run RSTP or MST.

-3- all the edge ports (i.e. port leading to endstations or routers) must be identified and configured for portfast.

If you fail to ensure those three conditions, you will be back to STP convergence time. In fact, it could be considered worse than STP because during the convergence (that will be just as long as for STP), you will potentially have more ports blocking in your network.

As long as all your switches are able to run RSTP or MST, I would not hesitate migrating, as the conditions are rather simple to meet.



paul.matthews Fri, 10/05/2007 - 13:03

I would only hesitate if I would end up with a mixed network - some rapid, some traditional - my experience has been that a mix-and-match approach would need to be quite carefully planned. Other than that, rapid all the way.

lamav Mon, 10/08/2007 - 06:01

Thanks, folks.

Here is what Cisco recommendsw] regarding uplinkfast and backbone fast, which as you know are embedded as default add-ons in rapid STP. So, could this also be a constraint to deploying rapid STP?

1 Do not enable Uplink Fast on access switches that have more than 2 uplinks.

2 Enable BackboneFast on all switches running STP only if they can all support BackboneFast, too.

In other words, dont deploy rapid STP if it is determined that you should not be deploying uplink and backbone...?

Francois Tallet Mon, 10/08/2007 - 08:59

-1- does not make sense (It could be indeed a recommendation from Cisco, as there are many people clueless about STP there too;-)

-2- is indeed the recommendation.

Uplinkfast is indeed integrated in RSTP. Backbonefast is really a feature from the past, and RSTP (or MST, I make no difference between them here) does waaay better. Again, if you can have your whole network running RSTP, there is no comparison with STP. If you need to do a mix STP/RSTP, or if you have shared segments, then only you need to think a little bit more about the pros and cons.



onurcoskun Mon, 10/08/2007 - 06:51

By default a switch operates in pvst+ using 802.1d stp. Therefore rstp cannot be used until a different spanning tree mode (mst or rpvst+) is enabled.

Also pay attention to the fact that rstp is an underlying mechanism that mst or pvst+ can use.

If your network fully operates with point to point links in full duplex mode, i suggest it would be best to implement stp. If the network doesnt match the criteria mentioned above, you cant implement rstp anyway.

Hope that helped.


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