Normal BPDU gets discarded if the message age field reaches max age.

I'd like to know if it's the same for TCN BPDUs on their way up to the root bridge.


Note that true TCN BPDUs were only used in the original STP. RSTP uses RST BPDUs with the TC bit set which have message/max age fields and are discarded when aged out.

True TCN BPDUs have no age fields, but as far as I understand it, they're not forwarded. A bridge receiving a TCN BPDU may (and probably will) decide to send TCN BPDUs on its own though.


The Topology Change Notification BPDU doesn't have a message age field. This is the format from the IEEE 802.1D-2004 standard:

  1. The Protocol Identifier is encoded in Octets 1 and 2 . It takes the value 0000 0000 0000 0000.
  2. The Protocol Version Identifier is encoded in Octet 3 . It takes the value 0000 0000.
  3. The BPDU Type is encoded in Octet 4. This field takes the value 1000 0000 (where bit 8 is shown at the left of the sequence). This denotes a Topology Change Notification BPDU.

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