Designated port selection

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

Designated port selection is the spanning-tree procedure for choosing the forwarding port for each network segment.

Designated port selection is the STP step that chooses one forwarding port for each network segment, helping create a loop-free topology. Each segment has exactly one designated port; other competing ports become non-designated and are blocked. section 14.3 section 14.3.3

Apply the selection rules in this order:

• Choose the port on the switch with the lowest root cost. section 14.3.3
• If root costs tie, choose the port on the switch with the lowest bridge ID (BID). section 14.3.3
• Remember that BPDUs are sent only through designated ports; root-bridge ports are designated, as are ports connected to root ports. section 14.3.3

Comparing two connected switches

When SW1 has root cost 4 and SW2 has root cost 8, SW1 G0/3 becomes designated and SW2 G0/0 becomes non-designated. If root costs tie, compare BIDs: SW4’s lower BID makes SW4 G0/1 designated. section 14.3.3

Figure 14.13 Each segment must have exactly one designated port. All ports on the root bridge are designated, and so are all ports connected to a root port. One designated port is selected on each remaining segment, and the remaining ports are non-designated.

Use the topology figure to identify the single designated port on each segment and distinguish blocked non-designated ports. section 14.3.3

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes

Explains root-cost comparison and BID tie-breaking, then connects the result to BPDU transmission and blocked ports.

Examples

Uses SW1, SW2, and SW4 scenarios to demonstrate root-cost comparison and BID-based tie-breaking.

What you can do after

Evaluate STP topologies by identifying designated ports, blocked ports, and the tie-breakers that determine them.

How it connects

Connects STP algorithm, root bridge election, root port selection, root cost, bridge ID, BPDUs, and non-designated ports.

liveBookmanning.com

Read Acing the CCNA Exam: Volume 1 Fundamentals and Protocols on liveBook Get the book at manning.com

Related concepts

• root costuses · Root cost is the cumulative spanning-tree path cost from a switch through a port to the root bridge.
• bridge identifieruses · A bridge identifier, or bridge ID, is a 64-bit value that identifies a switch within a spanning-tree instance and is advertised in bridge protocol data units.
• spanning tree protocolpart of · Spanning Tree Protocol is a Layer 2 protocol that prevents bridged-network loops by selecting a loop-free topology and blocking redundant paths.
• root portcontrasts with · A root port is the single port selected on a non-root switch to provide its active path toward the root bridge.
• designated portconfigures · A designated port is an STP port in the forwarding state that provides the preferred path away from the root bridge for a network segment.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.