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Tuesday, April 12, 2011

what extent would spanning tree not be viable to use?

Qsolved Question: During a CBT Nugget video it mentions Spanning-Tree is really only effective to around 4-5 daisy chained switches. Wondering if this is the case and to what extent would spanning tree not be viable to use?

Qsolved Answer:

  • The STP is capable of handling any switched topology with a default maximum diameter of 18 or less switches (the limit of 18 is given by the fact that BPDUs are in fact stored only for max_age-message_age seconds where the message_age is a BPDU hop counter set to 0 on root bridge and incremented on each subsequent STP bridge). The statement that STP is effective only for 4-5 switches is, in my opinion, an overstatement. It protects any reasonable switched topology against bridging loops. An overly large switched domain would become problematic for another reasons, not just because of STP.

    Additional bit of knowledge - if memory serves, the default spanning-tree timers were based on some assumptions of a network with a diameter of seven switches. The diameter can (at least when I last looked) be changed, but rarely does there seem to be a need to - especially with RPVST being as quick as it is.

    As to your question about spanning-tree in general, it seems that the push now is to have routed interfaces between devices where possible. It used to be that switching was significantly faster than routing, and as gear improves that's not really the case anymore. I'm sure there are benchmarks somewhere that will show a slight difference in some uses but nothing to really be alarmed about. As you may have guessed or already seen, spanning-tree doesn't know your intentions and when you plug something into the network it can cause STP to reconverge, sometimes with less-than-desireable results - with routed interfaces, it's significantly easier to direct traffic flows. In fairness, there are still things than can go wrong in a primarily routed network, but in my experience they're generally easier to plan, predict, and manage.

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