TCN and BPDU

[Nader ibrahimi]

hello to everyone 

I am just a little bit confused about tcn and bpdu :

TCN and BPDU is both different message means these message will send distinct or TCN send attached to BPDU ?

[MHM Cisco World]

Bpdu is send periodic 

TCN send when  topology change 

It  not send together. 

MHM

[paul driver · Accepted solution]

Hello
bpdus are used between switch's to establish a loop free spanning-tree topology

in a stable switch network - non root switches should only receive bpdus on their root ports and not send them

in the event of a topology change TC in the network the switch that’s initiates the change will send a TCN via bpdu upstream towards the root switches- the root switch will in turn send out a TCA acknowledgement and further TCs to all downstream switches via bpdus thus informing them to shorten their mac address table aging time.

As a result of this the stp topology will eventually converge to become stable again 

so I would say BPDUs are definitely used for topology changes notifications-acknowledgements aswell for establishing and maintaining a loop free stp topology 

Note- in RSTP uses different method of notifying a topology change as such TCNs are not used but again bpdus are.

[Joshqun Ismayilov]

TCN (Topology Change Notification) and BPDU (Bridge Protocol Data Unit) are different messages.

TCN is a separate message sent by a switch to notify about topology changes.
BPDU (Configuration & TCN BPDU) is used for spanning tree operations.
TCN BPDU is sent separately by non-root switches, but the Root Bridge includes topology change info in its BPDU.

Thanks,
Joshqun Ismayilov

[paul driver]

Hello
So just to confirm you are saying TCNs /TCAs/TCs are not sent in BPDUs ?

[Joshqun Ismayilov]

TCN BPDU is not a normal Configuration BPDU; it's a special type of BPDU with a Topology Change Notification bit set.

Normal BPDU used to establish and maintain Spanning Tree topology.
BPDU with TCN Notifies the Root Bridge that a topology change has occurred.

Thanks,
Joshqun Ismayilov

[paul driver]

Hello @Joshqun Ismayilov 
Okay - so semantics of a bpdu but would you agree all in all it’s still a bpdu for either 802.1d /w/s that carry's these TCNs,A,Cs

and TBH i was replying to the original statement from @MHM Cisco World that TCNs are not sent in BPDUS which we now all agree are.... good discussion though..

[MHM Cisco World]

I see my name' who call me LOL..

I dont mention sent in 

I say send together.

Anyway @Nader ibrahimi always use wireshark' if ypur lab dont have wireshark there is alot of capture in internet 

 this TCN frame.

MHM

[Martin L]

see https://community.cisco.com/t5/switching/generating-bpdus-in-rstp/m-p/5145465

Regards, ML
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[Devaa]

Already Paul explained it very well. Adding the summary in my words.

CONF BPDU in 802.1D

• Sent by Root Bridge via Designated ports every 2 secs. All the downstream non-root switches will receive that and will further forward it downstream via Designated ports every 2 secs. 

TCN BPDU in 802.1D

• When port goes down or port comes up in FWD state, TCN will be sent to Root Bridge. 
• Non-Root Switch will send TCN only once towards the Root Bridge telling something change, but no other info. 
• Root Bridge will inform all other switches telling something changed by sending Topology Change along with Conf BPDU by enabling TC and TCAck (only with first Conf+TC BPDU) bit for 35 Secs = 15 sec Fwd delay + 20 sec Max age
• When Conf+TC BPDU is received, Switch will set Mac Address table aging time from 300 sec (default) to 15 secs (fwd delay) and back to 300 secs to forget any mac quickly which is no more learned via that port to avoid loop.

802.1w RSTP

• Only one BPDU. No TCN BPDU. No more sending to Root Bridge. 
• All the switch will forward the Conf+TC for 2x hello timer which is 4 secs tc_while timer
• When a TC received via some port, it will flush the mac address immediately in all the ports, except from where TC was received, to avoid loop

[Nizamudheen]

This is a separate type of BPDU used to inform other switches for topology changes. TCN BPDU type field (0x80) while Configuration BPDUs (0x00). try to capture in Wireshark .

[M02@rt37]

@Nader ibrahimi 

In short, TCN and BPDU are distinct messages, but the TCN uses a special type of BPDU to notify other devices in the network of a topology change.