Solved: bpdu

sarahr202 · ‎07-08-2008

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:

cost=1 cost=2

sw1-----------sw2-----------sw3

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 !

kerek · ‎07-09-2008

Hi,

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:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml

Hope it helps, rate if does

Krisztian

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Francois Tallet · ‎07-10-2008

Hi Sarah,

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.

Regards,

Francois

View solution in original post

kerek · ‎07-09-2008

Hi,

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:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml

Hope it helps, rate if does

Krisztian

sarahr202 · ‎07-10-2008

thanks for your reply.

To help make my point, i use the following example

sw1---------------sw2

f0/1 f0/1

sw1 is configured as root.

sw2 receives bpdu from sw1 on f0/1 which is the root port.

If sw2 does not send any bpdu to sw1, then (1)what is the point having max-age timer on sw1's f0/1 since sw1 is not expecting any bpdu from sw2?

(2)how will sw1 determines if the sw2 is still functional without any bpdu from sw2,?