Wireless effect on spanning tree diameter

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Nov 16th, 2007
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Greetings All


I have a network with many Aironet BR350's, which are being phased out, but in the mean time I think we are going to hit the Spanning Tree diameter limits very soon. The situation is we have a switch diameter of 4 (switches only) the two bridges and another switch. I count each bridge as a switch I am at a diameter of 7. My ultimate question is how does a wireless bridge or access point affect the switching diameter?


Thanks...

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ericgarnel Fri, 11/16/2007 - 06:01
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Going out on a limb here... its' been a while since I played with the 350s

Do the BR350s participate in STP? Probably not, so their effect on diameter is most likely comparable to that of linking the two switches together


referring to http://www.cisco.com/warp/public/473/122.html


The diameter is completely dependent on the network design. The diameter is the maximum number of switches that you cross in order to link any two switches in the bridged network (which includes source and destination), if you assume the worst cases. You do not cross the same switch twice when you determine the diameter


Thus:

*


hello-The hello time is the time between each bridge protocol data unit (BPDU) that is sent on a port. This time is equal to 2 seconds (sec) by default, but you can tune the time to be between 1 and 10 sec.

*


forward delay-The forward delay is the time that is spent in the listening and learning state. This time is equal to 15 sec by default, but you can tune the time to be between 4 and 30 sec.

*


max age-The max age timer controls the maximum length of time that passes before a bridge port saves its configuration BPDU information. This time is 20 sec by default, but you can tune the time to be between 6 and 40 sec.


Each configuration BPDU contains these three parameters. In addition, each BPDU configuration contains another time-related parameter that is known as the message age. The message age is not a fixed value. The message age contains the length of time that has passed since the root bridge initially originated the BPDU. The root bridge sends all its BPDUs with a message age value of 0, and all subsequent switches add 1 to this value. Effectively, this value contains the information on how far you are from the root bridge when you receive a BPDU.


here is an example of an edge switch cost

#sh spanning-tree vlan 1 bridge


Hello Max Fwd

Vlan Bridge ID Time Age Dly Protocol

---------------- --------------------------------- ----- --- --- --------

VLAN0001 49153 (49152, 1) 0012.43e1.3b00 2 20 15 ieee




Theoretically, if the wireless bridge does not participate in STP, then it should behave like a physical link and normal STP rules should apply. Having said that, I have always followed the practice of not changing the default STP values.


The best advice I can give is to try it and see if it works or not.

melcara Fri, 11/16/2007 - 06:35
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If I follow what you are saying correctly, if the two bridges are not participating in spanning tree I can treat them as a single cable?

ericgarnel Fri, 11/16/2007 - 06:41
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Technically, yes.

A good test would be to set it up both ways:

wireless and then wired and compare the STP values to determine the difference and/of effect

if any

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