BPDU

Bridge Protocol Data Units:
Data frames that carry special information about bridge ID's and root path costs. The bridge sends a BPDU with the MAC of the port as source, and destination of STP multicast 01:80:C2:00:00:00.
Three types of BPDU's exist:

• Configuration BPDU (CBPDU) - used for Spanning Tree Computation
• Topology Change Notification (TCN) BPDU - announce changes in network topology
• Topology Change Notification Acknowledgment (TCA)

BPDU's are exchanged every 2 seconds by default, network changes, starting and stopping port forwarding is notified to switches.
Once a new device is attached to a switch, it will process BPDU's to determine the topology of the network. A host attached to a switch, it will go into a forwarding state, after it listens and learns for 30 seconds and the forward delay has reached it 15 second default time. Another switch would remain in blocking mode if a loop would arise from its presence in the network. TCN will inform other switches of a port change, injected into the network by a non-root switch and propagated to root. When this TCN is received a Topology change flag is set and sent in a BPDU, which then is sent to all other switches, instructing them to age out their forwarding table entries.
Fields:
Bridge ID (BID) is eight bytes in length - first 2 are Priority, the last 6 are MAC. If MAC reduction is used, the first 4 bits are priority, and the last 12 bits are VLAN ID or MSTP instance number.

Once there is a stable switched network topology, we should see the following:

• A MAC address for each switch
• Path cost to root associated with each switch port
• Port ID (MAC) with each switch port
  

Configuration BPDUs have:

• A unique Identifier of the switch that the transmitting switch believes to be root
• Cost of path to root from transmitting port
• The ID of transmitting port
  

Configuration BPDUs communicate and compute STP. A MAC frame showing a BDPU sends teh switch group address to dest address field. All connected switches receive the BPDU - the info is not forwarded, but calculated by receiving switch and topology info is taken into consideration.

When a BPDU exchange occurs:

• One switch is elected as root
• The shortest distance to root is calculated for each switch
• A designated switch is selected - the switch closest to root switch
• A port for each switch is selected (to the root)
• Ports in STP are selected

New BPDU:

With the introduction of RSTP, BPDUs have changes their functionality. 2 flags were previously used (TC and TCA). RSTP uses all six bits of the flag to:

• Define role and state of port the originated the BPDU
  

• Handle proposal and agreement mechanism

Legacy equipment using old BPDUs drop new BPDUs

How New BPDUs are Handled:

BPDU are Sent Every Hello-Time

BPDUs are sent every hello-time, instead of being relayed, every 2 seconds even if it does not receive any BPDUs from the root

Faster Aging of Information

If a hello is not received three consecutive times the timeout is expired. A neighbor is lost if this timer expires. In the previous version the problem may have existed anywhere on the link.

Note: Failures are detected even much faster in case of physical link failures.

Accepts Inferior BPDUs

This is similar to BackboneFast technology. The bridge receives inferior information from designated or root bridge, it is immedietly accepted and replaced of previously stored one.

C is aware of an existing root path, sends a BPDU to B, which contains info about the root bridge. B now does not send its BPDUs and accepts the port that makes bridge C the new root port.