We’ve already learned that on the shared Ethernet segments the switch with best path to the reach root switch is placed in the forwarding state. That switch is called the designated switch and its port known as designated port. In the order to the avoid loops, the non-designated port on other end of the link is placed in the blocking state.
The designated switch
is determined based on the following criteria are:
1. the switch with
lowest cost to the reach the root becomes designated switch on that link.
2. in case of tie, the
switch with lowest BID become designated switch.
Consider the
following examples are:
SW1 has lowest BID &
has been selected as root switch. SW2 and SW3 have then the determined their
own root port to the reach root switch. On the shared networks segment between the
SW2 and SW3 designated port needs to be the selected. Because SW3 has the lower
cost to the reach root switch (4<19), it Fa0/2 port will the designated port
for segment. The Fa0/2 port on the SW2 will be placed in the blocking state.
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