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The first decision that all switches in
the network make, is to identify the root bridge. The position of the
root bridge in a network will affect the traffic flow.
When a switch is turned on, the
spanning-tree algorithm is used to identify the root bridge. BPDUs are sent out
with the Bridge ID (BID).
 The BID
consists of a bridge priority that defaults to 32768 and the switch base
MAC address.
 By default
BPDUs are sent every two seconds.
When a switch first starts up, it assumes
it is the root switch and sends “inferior” BPDUs. These BPDUs contain
the switch MAC address in both the root and sender BID.
 All switches
see the BIDs sent. As a switch receives a BPDU with a lower root BID it
replaces that in the BPDUs that are sent out. All bridges see these and
decide that the bridge with the smallest BID value will be the root
bridge. 
A network administrator may want to
influence the decision by setting the switch priority to a smaller value
than the default, which will make the BID smaller. This should only be
implemented when the traffic flow on the network is well understood.
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Lab Activity
Lab Exercise: Selecting the Root Bridge
In this lab, the student will create a basic switch
configuration and verify it and determine which switch is
selected as root switch with factory default settings.
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Lab Activity
e-Lab Activity: Selecting the Root Bridge
In this lab, the following functions will be performed.
Verify configuration of hosts and switch by testing connectivity .
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