When switch send spanning tree updates those updates are sent in BPDUs
(bridge protocol data units). The BPDU will have a source mac address
associated with the originating switch/VLAN number. If a switch recieves
multiple BPDUs that indicate the same root cost it will pick the one which
came from the switch with the lowest (I'm pretty sure it's lowest and not
highest but I may be wrong) MAC address. There is also a port priority
which I believe (I'm not sure) can be configured to aid in the selection of
the root port
At 09:01 AM 02/06/2001 -0600, Jim Dixon wrote:
>Hi Nathan,
>
>Have you read Radia Perlman's Interconnections. There are two.
>The second edition I believe is the latest.
>She wrote spanning tree. This book does cover it in detail.
>ISBN# 0201634481
>
>At the time I looked Amazon had a used one in GOOD condition for 15 bucks.
>(FYI)
>
>
>-----Original Message-----
>From: Miller, Nathan - BSC [mailto:[EMAIL PROTECTED]]
>Sent: Tuesday, February 06, 2001 8:09 AM
>To: [EMAIL PROTECTED]
>Subject: Spanning Tree Question - Root Port Selection
>
>
>I have been looking for a while for further documentation of the process by
>which a switch selects its root port. Most of the books that I have
>searched for this information say something similar to the following quote
>from a CCO page: "A bridge's root port is the port through which the root
>bridge can be reached with the least aggregate path cost, a value that is
>called the root path cost." My problem is that they all seem to stop there.
>My question is this. If the root path cost is the same on multiple switch
>ports, how does STA determine which is the root port? Does it follow the
>same course as it would when selecting a designated port (root bridge, root
>path cost, sender ID, sender port).
>Many thanks for your thoughts.
>Nathan Miller
>
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Tom Pruneau
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