Bridge Protocol Data Units are how the bridges talk to each other, like
"hello" packets in routing protocols. They can be thought of as control
frames, versus data frames.
The reason there is no loop is because the data portion of the bridge or
switch interface is blocked from a DATA perspective only.
What the BPDUs verify is the path to the root bridge, so that if a broken
link in the Spanning Tree is detected, the disabled port can be opened for
data if it is elected to do so when the tree is recalculated.
A very good pdf document on Spanning Tree comes directly from Cisco
itself...I have the link for it in the office, and I'll post it tomorrow.
for troubleshooting spanning tree, see
http://www.cisco.com/warp/public/473/16.html
----- Original Message -----
From: Brian M. Green <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Monday, March 19, 2001 12:46 PM
Subject: Spanning Tree
> In reading about Spanning Tree, I am confused about BPDU traffic. Once
> a root is selected, do all switches still send BPDUs out all
> interfaces? I know that redundant links are put in a blocking state so
> that no traffic is sent or received. I am reading Webb's BCMSN book by
> Cisco Press and it states that "Receiving BPDUs on multiple ports
> indicates that it has a redundant path to the root bridge". This seems
> to indicate that only the root is sending BPDUs. Later, it says "Those
> ports placed in a blocked state will continue to send and receive BPDU
> information but will not be allowed to send or receive data." These two
> statements seem contradictory in the fact that it seems you could
> receive BPDUs from a neighboring switch port that is blocked but still
> sending BPDUs, and also receive BPDUs from the root, so that you are
> receiving multiple BPDUs but there is no loop. Could someone please
> explain how this works to me?
>
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