hi every body!
I need you guys to help me this:
sw1,sw2,sw3 are three switches.
sw1 is root bridge connected to sw2 via f0/2 and sw2 is connected via f0/2.
sw1 is connected to sw3 via f0/3 and sw3 is connected via f0/3
sw2 is connected to sw3 via f0/1 and sw3 is connected to sw2 via f0/1.
Let say sw3 lost the connection on f0/3(root port ) which is connected to the root brige sw1.
Sw3 starts sending bpdu claiming to be root bridge on f0/1 connected to sw2.
sw2 receives the inferior bpdu from sw3 on its blocked port f0/1.
My question in next 2 second, sw2 will send bpdu with root bridge id=sw1 mac, to sw3 on f0/1 that means sw2 will know the better rootbridge (sw1).
while studying about backbone fast, i find out:
sw2 receives the inferior bpdu from sw3 claiming to be root, sw2 ignores it for max-age timer, then transmits its bpdu claiming sw1 being the root bridge
My question why sw2 has to wait 20(max-age timer) before it could transmits its bpdu, considering the fact every bridge transmits bpdu every 2 seconds?
thanks a lot and have a nice day !
Lets suppose a problem in your own scenario. S1 is root &connected to both S2 & S3. s2 & S3 are in turn connected. Now, S3 doesnt recv a BPDU from root S1. Then S3 sends a inferior BPDU to S2 & S2 waits max-age.
The reason S2 waits for max-age to expire is, there may be cases when the link between S1 & s3 is congested due to which s3 misses 1 or 2 BPDU's from S1.but may later start recvng them properly. So to differentiate between the exact failure of link & the misses due to congestion, STP was designed to wait for max age to expire before informing the other switch of alternate path.
The below link will explain all!