Configuration BPDUs are sent by LAN switches and are used to communicate and compute the Spanning Tree topology. After the switch port initialises, the port is placed into the Blocking state and a BPDU is sent to each port in the switch. By default, all switches initially assume that they are the Root of the Spanning Tree, until they exchange Configuration BPDUs with other switches. As long as a port continues to see its Configuration BPDU as the most attractive, it will continue sending Configuration BPDUs. Switches determine the best Configuration BPDU based on the following four factors (in the order listed):
- Lowest Root Bridge ID
- Lowest Root path cost to Root Bridge
- Lowest sender Bridge ID
- Lowest sender Port ID
The completion of the Configuration BPDU exchange results in the following actions:
- A Root Switch is elected for the entire Spanning Tree domain
- A Root Port is elected on every Non-Root Switch in the Spanning Tree domain
- A Designated Switch is elected for every LAN segment
- A Designated Port is elected on the Designated Switch for every segment (all active ports on the Root Switch are also designated)
- Loops in the network are eliminated by blocking redundant paths
NOTE: These characteristics will be described in detail as you progress through this module.
Once the Spanning Tree network has converged, which happens when all switch ports are in a Forwarding or Blocking state, Configuration BPDUs are sent by the Root Bridge every Hello time interval, which defaults to two seconds. This is referred to as the origination of Configuration BPDUs. The Configuration BPDUs are forwarded to downstream neighbouring switches via the Designated Port on the Root Bridge.
When a Non-Root Bridge receives a Configuration BPDU on its Root Port, which is the port that provides the best path to the Root Bridge, it sends an updated version of the BPDU via its Designated Port(s). This is referred to as the propagation of BPDUs.
The Designated Port is a port on the Designated Switch that has the lowest path cost when forwarding packets from that LAN segment to the Root Bridge.
Once the Spanning Tree network has converged, a Configuration BPDU is always transmitted away from the Root Bridge to the rest of the switches within the STP domain. The simplest way to remember the flow of Configuration BPDUs after the Spanning Tree network has converged is to memorise the following four rules:
- A Configuration BPDU originates on the Root Bridge and is sent via the Designated Port.
- A Configuration BPDU is received by a Non-Root Bridge on a Root Port.
- A Configuration BPDU is transmitted by a Non-Root Bridge on a Designated Port.
- There is only one Designated Port (on a Designated Switch) on any single LAN segment.
Figure 31.2 below illustrates the flow of the Configuration BPDU in the STP domain, demonstrating the four simple rules listed above:
Figure 31.2 – A Configuration BPDU Flows throughout the STP Domain
- Referencing Figure 31.2, the Configuration BPDU is originated by the Root Bridge and sent out via the Designated Ports on the Root Bridge towards the Non-Root Bridge switches, Switch 2 and Switch 3.
- Non-Root Bridge Switch 2 and Switch 3 receive the Configuration BPDU on their Root Ports, which provide the best path to the Root Bridge.
- Switch 2 and Switch 3 modify (update) the received Configuration BPDU and forward it out of their Designated Ports. Switch 2 is the Designated Switch on the LAN segment for itself and Switch 4, while Switch 3 is the Designated Switch on the LAN segment for itself and Switch 5. The Designated Port resides on the Designated Switch and is the port that has the lowest path cost when forwarding packets from that LAN segment to the Root Bridge.
- On the LAN Segment between Switch 4 and Switch 5, Switch 4 is elected Designated Switch and the Designated Port resides on that switch. Because there can be only a single Designated Switch on a segment, the port on Switch 5 for that LAN segment is blocked. This port will not forward any BPDUs.