About this interactive
The lessons' examples, as one method:
1. **Root bridge:** lowest bridge ID. Priority is compared first; the MAC address breaks a tie.
2. **Root path cost:** add the cost of each link on the way to the root (1 Gb/s = 4, 100 Mb/s = 19).
3. **Root port:** each non-root switch's lowest-cost path. Ties go to the lower sender bridge ID, then the lower sender port ID.
4. **Designated port:** on each link, the end with the lower root path cost. Ties go to the lower bridge ID. Every port on the root bridge is designated.
5. **Blocked (alternate) ports:** any port that is neither root nor designated.
What you'll learn
- Trace spanning tree on a multi-switch topology: elect the root bridge, then find each switch's root port and each link's designated port
- Apply the tiebreakers, lower sender bridge ID for root ports and lower bridge ID for designated ports, and identify the ports left blocked
Aligned to
Cisco CCNA
2.5
Interpret basic operations of Rapid PVST+ Spanning Tree Protocol
Key terms
- Root Bridge
- The switch elected as the reference point for the Spanning Tree Protocol topology, determined by the switch with the lowest Bridge ID (priority + MAC address). All other switches calculate their best path to the root bridge and block redundant links to create a loop-free tree.
- Root Port
- The port on a non-root switch that provides the least-cost path to the root bridge.
- Designated Port
DP
- In STP, the port on each network segment that provides the best path to the root bridge and is placed in a forwarding state. Each segment has exactly one designated port, which is selected based on the lowest cumulative path cost to the root.
- Alternate Port
- A switch port that is not elected as a root port or designated port and is placed in a blocking state to prevent loops, serving as a backup path.
- Bridge ID
BID
- A unique identifier used in the Spanning Tree Protocol consisting of a configured priority value and the bridge's MAC address. The BID is used in STP elections to determine which switch becomes the root bridge.
- Root Path Cost
- The cumulative cost calculated by a switch to reach the root bridge, based on the speed of each link along the path.
- Bridge Priority
- A configurable numeric value within the Bridge ID, defaulting to 32768 and incrementing in multiples of 4096, used to influence which switch is elected as the root bridge in STP.
Topics
Stp
Root Bridge
Root Port
Designated Port
Alternate Port
Port Roles
Interactive
Hotspot
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