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RE: Root Bridge Lab from Sybex [1:7732] posted 02/20/2002
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The selection of the root switch for a particular VLAN is very important.
You can choose it (by changing the switch priority,  by advertising  their
priority and the MAC address. Or you can let the switches decide on their
own.The second option is risky because there may be sub-optimal paths in
your network if the root selection process is not controlled by you.

In order to avoid loops, it is imperative that a switch does not have
redundant physical path to the root bridge,
since the switching universe revolves around it. For STP to work correctly,
BPDUs are sent and received. Here are the rules that govern the operation of

When the switches first come up, they start the root switch selection
process by each switch transmitting BPDU to its directly connected switch on
a per-VLAN basis.

As the BPDU goes out through the network, each switch compares the BPDU it
sent out to the one it received from its neighbors. From this comparison,
the switches come to an agreement as to who the root switch is. The switch
with the lowest priority in the network that wins this election process.

STP Rule One: All ports of the root switch must be in forwarding mode
(except for some corner cases where self-looped ports are involved).
Next, each switch determines their best path to get to the root. They
determine this path by comparing the information in all the BPDUs received
on all their ports. The port with the smallest information contained in its
BPDU is used to get to the root switch; that port is called the root port.
After a switch figures out its root port, it proceeds to Rule Two.

STP Rule Two: Once a switch determines its root port, that port must be set
to forwarding mode.

In addition, for each LAN segment, the switches communicate with each other
to determine which switch on that LAN segment is best to use for moving data
from that segment to the root bridge. This switch is called the designated

STP Rule Three: In a given LAN segment, the designated switch's port that
connects to that LAN segment must be placed in forwarding mode.

STP Rule Four: All other ports in all the switches (VLAN-specific) must be
placed in blocking mode. This is only for ports that are connected to other
bridges or switches. Ports connected to workstations or PCs are not affected
by STP; they remain forwarded.

Hope that clears the concept.

-----Original Message-----
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx]
Sent: Wednesday, February 20, 2002 2:29 PM
To: associate@xxxxxxxxxxxxxx
Subject: RE: Root Bridge Lab from Sybex [1:7732]

Well, it doesn't really help..

What I am concerned about is how to decide on a designated port.. Mu
understanding is that there is one designated port PER LAN SEGMENT.

Is that right?

Can you refer to the Sybex question and tell me

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