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TOPIC: STP- Illustrated (Spanning Tree protocol)

STP- Illustrated (Spanning Tree protocol) 11 years 10 months ago #16492

  • p5chin
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Hi,

Found this site while googling and hope it would be of use here:

[code:1] www.cisco.com/warp/public/473/spanning_tree1.swf [/code:1]

- This is a simple illustration of how stp works.

Re: STP- Illustrated (Spanning Tree protocol) 11 years 10 months ago #16493

That is helpfull p5chin. I'm just getting into stp now.

One thing that wasn't answered though and I can't find it in what I have read so far, is how does the bridge get it's initial ID ? Is it it's mac address or some such ?

Re: STP- Illustrated (Spanning Tree protocol) 11 years 10 months ago #16501

  • Dove
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Yes Starfire,
Initialy the switch will take its own MAC address as its Bridge ID.

Re: STP- Illustrated (Spanning Tree protocol) 11 years 10 months ago #16504

I thought as much and this is what has been bugging me...

The MAC address does not necessarily denote the power/ability of the device so how does it work out which is the better spec device?

For instance, if you had an old/slow switch in the tree but it's organisational half of it's mac address was lower alphabetically than say your fresh out of the box, all bells and whistles, new device.

If the election is initially based alphabetically then the old one could get elected in this scenario. Or, is it all about hop speed and whoever has the better speeds gets elected regardless of specification ?

Re: STP- Illustrated (Spanning Tree protocol) 11 years 10 months ago #16505

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Each switch will have a Bridge ID Priority value, more commonly referred to as a BID. This BID is a combination of a default priority value and the switch's MAC address, with the priority value listed first. For example, if a Cisco switch has the default priority value of 32,768 and a MAC address of 11-22-33-44-55-66, the BID would be 32768:11-22-33-44-55-66. Therefore, if the switch priority is left at the default, the MAC address is the deciding factor

Thats why Cisco prefer to change the priority to least value than others. so that it will be act as a root bridge.

Re: STP- Illustrated (Spanning Tree protocol) 11 years 10 months ago #16507

Ahh.. click .. So you can manually intervene by adjusting the priority value.

Got it now. Thanks Dove
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