RSTP Max hop count

Unanswered Question

Hello, if there is anyone that can please provide any documentation supporting for or against this idea it would be great. There is a plan that has been proposed by the designers in my organization to use RSTP over LAN extensions to provide redundancy amongst buildings. The number of Cisco switches within the ring will be upwards of 12~20, and to be clear they will all be in a ring not a hieratical design. I know that there are much better ways, so please don't suggest alternatives, we want facts or articles for or against this idea.



Thanks.


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Giuseppe Larosa Thu, 04/23/2009 - 12:43
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Hello Terry,

the max-age has to be increased to take in account the worst case diameter when the ring is broken or the BPDUs will be discarded as expired.


see


http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=4416924&isnumber=4416743


ring topology is not recommended


I remember another thread about this

see


http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=LAN%2C%20Switching%20and%20Routing&topicID=.ee71a04&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40^1%40%40.2cc2be90/3#selected_message


you may need to copy the whole test in the browser to open the link


I've attached a copy of that paper


Hope to help

Giuseppe




Francois Tallet Thu, 04/23/2009 - 16:34
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The paper you mention is most likely BS, just like the document [1] they reference. They did their study with a bogus simulator. We had the opportunity to talk to those guys and after they admitted some problems with their simulator, they never retracted anything about those papers that are almost the only "research" hit that you get on STP. Very frustrating.

The only thing you need to be careful about is that your BPDU does not age out. The age is incremented by one at each step. The diameter recommended in the IEEE spec is too conservative and correspond to the use of an old software bridging running STP. With RSTP, just increase max age, it will not have impact on the convergence time if all your links are p2p (and they should).

As with any distance vector protocol, RSTP could get into a "count to infinity" situation if the root disappears from the network. In that case, communication could be affected until the old stale information is removed from the network. A matter of second indeed. There should be no temporary loop of traffic, only the control information can loop around. This situation cannot occur if the root bridge is part of the ring.

We have customers using RSTP on access rings for metro ethernet. We get decent convergence time of the order of the second. It's hard to get an exact figure as it depends on the number of mac addresses, the number of vlan and the location of the failure compared to the location of the blocked port.

Regards,

Francois

Giuseppe Larosa Fri, 04/24/2009 - 00:36
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Hello Francois,

I had seen this was just a work made with a simulator, so it is correct to say that it is important to avoid that BPDUs can age out.

Nice to hear that RSTP works well on a ring topology with acceptable convergence times.


Best Regards

Giuseppe


Francois Tallet Fri, 04/24/2009 - 08:28
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Hi Giuseppe,

Sure, I just wanted to stress that it was not only made with a simulator, but with a crappy one;-)

Regards,

Francois

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