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BPDU Message types and flags


I have been reading about the BPDU packet format and want to understand when the the different type and flags are sent.

Can you tell my why sometimes packets are sent with BPDU message type of Topology Change and other times they are Configuration BPDU but with the TC flag bit set?

What is the difference between TC message type and TC flag?

If you can explain or point me to a link I appreciate it.


Cisco Employee

Re: BPDU Message types and flags

Hello Bryan,

This link will answer your queries

Understanding Spanning-Tree Protocol Topology Changes



Re: BPDU Message types and flags

Whenever there is a change in the topology of network, theh change has to be communicated by the switches to the root. Switches generate a TCN & forward it to the root or another designated bridge in its path to the root. This designated bridge then forwards the same TCN further & the process continues till the root has received the BPDU with TCN. Please note that the designated bridge having recvd the BPDU with TCN, acknowledges the switch generating the mesages with a TCA (ack).

The root now floods the topology with a configuration BPDU which informs rest of the topology about the change.

hope that clarifies.

Cisco Employee

Re: BPDU Message types and flags

Configuration BPDUs -are originated by the Root Bridge and flow outward along the active paths that radiate away from the Root Bridge.

Topology Change Notification BPDUs - flow upstream (toward the Root Bridge) to alert the Root Bridge that the active topology has changed.

Type - Determines which of the two BPDU formats the frame contains (Configuration BPDU or TCN BPDU).

Type of BPDU can be one of the following

0x00 (Binary: 0000 0000) Configuration BPDU

0x80 (Binary: 1000 0000) Topology Change Notification (TCN) BPDU

Robert, A non root switch originates a TCN BPDU in two conditions:

=It transitions a port into the Forwarding state and it has at least one Designated Port.

=It transitions a port from either the Forwarding or Learning states to the Blocking state.

These situations construe a change in the active topology and require notification be sent to the Root Bridge. Assuming that the current bridge is not the Root Bridge, the current bridge begins this notification process by sending TCN BPDU out its Root Port. It continues sending the TCN BPDU every Hello Time interval seconds until the TCN message is acknowledged

Now the second part of the question :

Difference between TC message type and TC flag

Flags- are Used to handle changes in the active topology and is a field of COnfig BPDU

Flags can be :

LSB = Topology Change (TC) flag

MSB = Topology Change Acknowledgment (TCA) flag

In a nutshell, the FLAG is set to Topology Change (TC) by the Root Bridge/Switch to notify the downstream bridges/switches who are involved and need to know regarding the change.


Parikshit Sharma