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Designated bridge

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

A designated bridge is the switch that has the designated port for a particular network segment.

A designated bridge is the switch with the designated port for a particular network segment. Every segment has exactly one designated port, which forwards traffic away from the root bridge. The term applies to both STP and RSTP. section 15.2.3

Use the designated-bridge status to distinguish port roles:

  • A switch that is not the designated bridge for a segment can have an alternate port when that port is neither root nor designated. section 15.2.3
  • A designated bridge can have a backup port when multiple ports on the same switch connect to the same segment. section 15.2.3

Reading spanning-tree roles

In the sample output, SW3 G0/0 is designated and forwarding, while SW3 G0/1 is backup and blocking. SW1 G0/1 is alternate because SW1 is not the designated bridge for that segment.

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

How deep it goes
Defines ownership of the designated port and traces alternate and backup consequences without detailing election calculations.
Examples
Uses shared-segment diagrams and show spanning-tree output showing designated, root, alternate, and backup roles.
What it assumes
Assumes basic understanding of network segments, root bridges, designated ports, and spanning-tree port roles.
What you can do after
Diagnose with Cisco show spanning-tree by identifying designated, alternate, backup, and root port roles.
How it connects
Connects designated bridges with root bridges, designated ports, root ports, alternate ports, backup ports, STP, and RSTP.
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Related concepts

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.