Spanning Tree Path Cost Calculationn

[cen...@pacbell.net]

All,

I am curious exactly how the path cost is calculated for spanning
tree.

ROOT-SWITCH A (COST 4-->)----------(<--COST 5) SWITCH B.

As switch B receives the BPDU from ROOT SWITCH A, will the cost be
sent out a 0 or sent out as 4? And when B gets the BPDU, does it
calculate the cost to root switchA as 5,4 or 9?

Thanks.

++central

[Thomas Kuborn]

I believe the cost sw_A (root) sends out is 0; then when sw_B receives the
BPDU, it will give the BPDU a cost of 5 since its interface has a cost of 5

Why would you want to have difference STP costs at the 2 ends of the same
link ?

Thomas

<cen...@pacbell.net> a écrit dans le message de news:
85ac2a79.03050...@posting.google.com...

[Sam Wilson]

In article <3eb94603$0$11154$ba62...@reader0.news.skynet.be>, Thomas
Kuborn <thomas...@softhome.net> wrote:

> <cen...@pacbell.net> a écrit dans le message de news:
> 85ac2a79.03050...@posting.google.com...
> >

> > ROOT-SWITCH A (COST 4-->)----------(<--COST 5) SWITCH B.
>

> Why would you want to have difference STP costs at the 2 ends of the same
> link ?

Good question - doing that can cause interesting connectivity problems
but it can arise by default. Different manufacturers (and different
revs of Cisco s/w!) have different default values for port costs at
different speeds, and RSTP and MSTP, the new IEEE standards, seem to
have changed them all again. Whoopee!

Sam