Identifying Designated Ports STP

Identifying Designated Ports

The third step in the STP convergence process is to identify designated ports. A single designated port is identified for each network segment. This port is responsible for forwarding BPDUs and frames to that segment.

If two ports are eligible to become the designated port, then there is a loop. One of the ports will be placed in a blocking state to eliminate the loop.

Similar to a root port, the designated port is determined by the lowest cumulative path cost leading the Root Bridge. A designated port will never be placed in a blocking state, unless there is a change to the switching topology and a more preferred designated port is elected.

Note: A port can never be both a designated port and a root port. Consider the following example:

Ports on the Root Bridge are never placed in a blocking state. Thus, the two ports off of SwitchA will automatically become designated ports.

Remember, every network segment must have one designated port, regardless if a root port already exists on that segment.

Thus, the network segments between SwitchB and SwitchD, and between SwitchC and SwitchE, both require a designated port. The ports on SwitchD and Switch E have already been identified as root ports, thus the ports on Switch B and C will become the designated ports.