Selecting STP designated port (DP)

We’ve already learned that, on the shared Ethernet segments, the switch with the best path to reach the root switch is placed in forwarding state. That switch is called the designated switch and its port is known as the designated port. In order to avoid loops, the non-designated port on the other end of the link is placed in blocking state.

The designated switch is determined based on the following criteria:

  1. the switch with the lowest cost to reach the root becomes the designated switch on that link.
  2. in case of a tie, the switch with the lowest BID becomes the designated switch.

Consider the following example:

STP selecting designated port

SW1 has the lowest BID and has been selected as the root switch. SW2 and SW3 have then determined their own root port to reach the root switch. On the shared network segment between SW2 and SW3 a designated port needs to be selected. Because SW3 has the lower cost to reach the root switch (4<19), its Fa0/2 port will be the designated port for the segment. The Fa0/2 port on SW2 will be placed in blocking state.

If the link between SW1 and SW3 fails, STP will converge and the Fa0/2 port on SW2 will be placed in the forwarding state.

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