Fastest Spanning Tree – Best STP for Multiple-Ring Topology

ieee-802.1wspanning treeswitch

I have a topology in which there is a main ring and some nodes in the main ring have a minor ring.
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1- which STP configuration has the fastest convergence time? How would you configure spanning tree for all switches?
I think RSTP does the best, but Is there any other configuration to help convergence time?

2 – Does RSTP have any limitation? limitation In number of switches for instance? or incompatibility with other vendor's RSTP or spanning tree algorithm?

3- Is RSTP practical solution for this topology with a large number of switches? 45 switches as the main ring and more than 100 switches in minor rings?

4- Is there any other faster way, rather than spanning tree, to provide protection for this large network? 

Thank you all! 

Best Answer

  1. MSTP and RSTP converge equally fast, MSTP is the current protocol version. Given the topology in your diagram (ugh!) and its tree depth, no STP variant will converge quickly (if at all).
  2. xSTP default parameters are designed for a maximum tree depth or chain length of 20 hops. Building longer/deeper constructs requires parameter tuning or your network will never converge. Usually, trees are much more shallow with a maximum of four or five L2 levels, ideally only two or three.
  3. Generally, yes. You'll need to organize them better however. Don't build rings (at all). The best design approach in regard of performance, scalability and resiliance is two core switches, a distribution layer to increase the port count for the access layer, and an access layer where your nodes connect. When properly combining L2 and L3 connections, that approach scales up to tens of thousands of ports. At least the connections between core and distribution are redundant, often the distribution-access interlinks as well.
  4. That is a question too broad to answer. You can build a network in more than one way. xSTP is probably the easiest, less L2 and more L3 links scale better.

Edit:

the topology is not something that I can change, All the wirings have been done yet.

In case you've got multiple wires between closets you can still improve the topology significantly. Patching connections "through" = across intermediate enclosures allows you to reduce the tree depth significantly.

With just four links between closets (which is the absolute minimum we're deploying) you could e.g. elect Switch4 and Switch7(4) as "core" switches for separate L3 domains (you'll need L3 switches though). Make them redundant (stack) and connect all ring switches to the nearest stack. Run double L2 links between the domains and your network has significant improvements - it'll run xSTP without (too much) tweaking, has very few single point of failures, and increased throughput (depending on the exact details).

Make sure you run MSTP in (at least) two instances and configure the "core" switches as root bridges.

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