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Hi every body!
Thanks a lot for all of you who help people new to networking, understand the technology.
I need some help with following:
f0/1 f0/2 f0/3 f0/3
sw1 is root bridge.
sw2 receives the bpdu add 1(cost) and transmits on its port(f0/3) to sw3
Does sw3 add the cost 1 and transmit bpdu on its port(f0/3) to sw2?
thanks a lot !
SW1's 0/1 has an age timer, but it is not used on a designated port, as you guessed. It's only relevant when the port is storing better information it has received from a neighbor.
SW1 does not know if SW2 is still functional. It does not have to. Regardless of the state of SW2, SW1 forwards traffic on its port f0/1. Even if there was redundancy between SW1 and SW2, SW1 would not care about the health of SW2. SW2 would be responsible for blocking one port while SW1 would have both its ports forwarding.
In fact, SW1 may not even know that it is connected to a switch on its port f0/1 as SW2 might never ever send a BPDU on its root port.
The receiving switch does not send back BPDU on its root port (in sw3's case on f0/3). In case of topology change TCN is sent. Please refer to this link for further information:
Hope it helps, rate if does