STP Cbpdu

[parthrawat979]

So, in a topology like three switches connected with each other. Only the root bridge generates CBPDU. So  after shutting down sw3's it's e0/0(rp) link as per the rules SW3 will generate TCN and the root(sw1) should generate a cbpdu with tc ack flag on for the first time and then cbpdu flag on with tc flag on. But when I did capture the link I found that the cbpdu does have a root bridge id of switch 1 but the bridge id of sw2 why's that?? And how can one determine who's the real sender of a cbpdu cause as per the rules only root bridge generates the cbpdu.

[Giuseppe Larosa]

Hello @parthrawat979 ,

STP BPDUs are relayed but not transparently some fields are updated on each switch:

the path cost to the root :

starts a 0 out of the root bridge. It is incremented by the cost on the root port when relying

Bridge ID : reflects  the STP Bridge ID of the actual sender switch so yes it is correct to see the bridge ID of Sw2 on link between SW2 and SW3

port ID : port priority + the SNMP ifindex of the sending port on the actual sender switch

the need for this is to be able to compare BPDUs coming from different switches a Designated Port has to elected on each link/segment

Hope to help

Giuseppe

[parthrawat979]

How can I identify that whether the cbpdu is sent by SW1 as it's the root and only root generates the cbpdu so Sw1 must be the actual sender here??

[Giuseppe Larosa]

Hello @parthrawat979 ,

SW1 is the originator of the CBPDU not the actual sender in the segment betweeen SW2 and SW3.

just to make an example : 

how STP can detect which port is DP and which port is to be in blocking state when you connect ports 3 and 4 of SW 2 together ?

the STP BPDUs have several fields and some of them change at every physical port.

in this case port 3 is DP and port 4 will be blocked for having a lower port ID  .

Hope to help

Giuseppe

[balaji.bandi]

Where are the capture results? The results change depending on how the switches are

where is that capture results, the results change depends how the switches configured.

https://networklessons.com/spanning-tree/introduction-to-spanning-tree#Key_Takeaways

I suggest the post go to the Learning Cisco forum.

https://learningnetwork.cisco.com

[parthrawat979]

The capture results simply shows that in root bridge id is sw1 base mac but the bridge identifier shows the base mac of Sw2. Now the issue is when sw3 generates tcn then sw1 must send the cpbdu with tc and tcack flags on. But I can't identify the real sender of the cbpdu which I know is sw1 but how can you make that sure in the capture?? Since the bridge identifier shows the base mac of sw2.

[paul driver]

Hello
PVST
Tc is received by the Sw1 (root ) either from Sw2/SW3, in this instance by Sw2 which would receive a tc from Sw3 on it new RP 


RVPST
Either switch will send either proposal/agreement to each other 
I would say Sw1 (root switch) should not participate as it the root both both switches 2/3