STP costs

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Jan 26th, 2009
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One doubt: When calculating the cost of a nonroot switch to the root switch... do you add only the costs of outgoing interfaces on the path from the nonroot to the root or do you add the cost of all interfaces in every segment? I hope my question is clear.

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rais Mon, 01/26/2009 - 09:46
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The Cost is added to BPDUs coming inwards to the switch port.


Istvan_Rabai Mon, 01/26/2009 - 12:27
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Hi Francisco,

You should accumulate the costs of interfaces going on the path from the nonroot switch toward the root switch.

This basically expresses the cost of transmitting a packet to the root switch from a specific nonroot switch.



fjcardenas-1 Thu, 01/29/2009 - 06:35
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I am reading a book that shows an example were they add only the outgoing interfaces in the path from the nonroot to the root. That confused me.

So, what you really do is add all the costs of interfaces in the path to the root. For example: If You have SW Root ---- SW1 ----- SW2 (all with FastEthernet - considering default costs), then the cost from SW2 to Root would be 19+19+19+19?.. is that right?.

Or... SW2 receives a BPDU form SW1 with cost of 38, then the cost to reach Root would be 38 (received BPDU cost) + 38 (the cost of the two interfaces in the link from SW2 to SW1).... is that right?

rais Thu, 01/29/2009 - 07:51
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When a BPDU leaves Root SW, it has zero cost. When it enters SW1 the cost to root would be 19, when it enters SW2, the cost to root would be 19+19 = 38.

Incoming Direction = coming from root into SW1's Root Port which is FastEthernet [cost=19]in this case.



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