About this interactive
From the lesson, the four BPDU fields the election uses:
- **Root ID:** the bridge ID of the switch the sender believes is the root. A switch starts with its own and changes it when it hears a lower one; once settled, every switch sends the same root ID.
- **Root path cost:** the sender's cost to reach the root. The root sends 0; each switch adds the cost of the port the root's BPDU arrived on.
- **Bridge ID:** the sender's own priority, extended system ID (VLAN) and MAC. It never changes, whichever switch is root.
- **Port ID:** the port priority (default 128) and interface number of the port the BPDU leaves from, so it differs on every port.
What you'll learn
- Match what a switch puts in each BPDU field: root ID, root path cost, bridge ID and port ID
- Explain which fields stay the same and which change as BPDUs are learned, passed on and sent out different ports
Aligned to
Cisco CCNA
2.5
Interpret basic operations of Rapid PVST+ Spanning Tree Protocol
Key terms
- Bridge Protocol Data Unit
BPDU
- A data message exchanged between switches running Spanning Tree Protocol to detect network loops and elect a root bridge. BPDUs contain the sending switch's BID and path cost information used to build the loop-free spanning tree.
- 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.
- Port ID
- A two-byte STP identifier assigned to each switch port, consisting of a priority value and an interface number, used as a tiebreaker when selecting a root port.
- 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.
- 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.
- Root Port
- The port on a non-root switch that provides the least-cost path to the root bridge.
Topics
Stp
Bpdu
Bridge Id
Root Path Cost
Port Id
Interactive
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