From: Anthony Pace (anthonypace@xxxxxxxxxxx)
Date: Wed Jul 24 2002 - 14:49:27 GMT-3
Tony,
I also read all the CCO stuff about this (which basically said they are
both used to help determine which link to use to reach the root) and
also posted about this + did some testing myself. This is what I have
accumulated:
The link with the lowest cost would be the winner when deterrmining
where to forward/block if redundant paths exist to the root. This is
the cost of the link based on the bandwidth of the media.(I believe
that it would also accumulate costs from other links between this link
and the root) You override this with the SET SPANTREE PORT COST.
The priority is kind of a tie breaker and is determined by the priority
set on the device on the other end of the link + the local ports number
(literal number, meaning 7 is lower that 8 etc...)In other words if the
ROOT BRIDGE was on port 1 and 2 (redundantly)and the ROOT BRIDGE had
set the link attatched to your port 1 to a low number, then you would
see a lower priority on port 1. SO your setting of the priority on a
port infuences the priorty decision of the device on the other side of
the link. Your port number is also considered as a tie breaker. This I
observed, and have not seen it written about much anywhere, so it might
need to be verified.
It seems like the blocking/forwarding always happens on the switch
downstream of the ROOT, meaning that if 1 was ROOT and 2 hung off 1 and
3 hung off 2: 1=>2=>3. 2 would block/forward going to 2 and 3 would be
the switch blocking/forwarding going to 2.
The other related commands are the SET SPANTREE PORTVLANPRI and
PORTVLANCOST which are for ISL trunk ports, and let you tweek it per
VLAN. (Which is actually very powerfull because you can put half the
VLANS on on link and half on another,thereby having load balancing +
redundancy)
SET SPANTREE PRI is to determine the ROOT BRIDGE (per VLAN on ISL).
Someone correct me where I am worng (especially the with respect to the
PORT PRIORITY)
Anthony Pace
On Wed, 24 Jul 2002 13:15:41 +0800, "Tony Pang" <tonypang@dmx.com.hk>
said:
> Hi,
>
> I am confused about the STP port priority and port cost. What are their
> difference? The manuals doesn't state the difference. I want to find
> the
> IEEE documents but I can't find the links to download IEEE documents in
> the
> getieee802 page. Anybody can help me?
>
> Best regards,
> Tony Pang
> DMX Technogloies (HK) Ltd.
> tonypang@dmx.com.hk
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