backbone fast

Jul 8th, 2008

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.

f0/1 f0/1

sw2-------------sw3

b d

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.

Overall Rating: 5 (3 ratings)

Replies

sarahr202 Wed, 07/09/2008 - 19:17

thanks a lot Andrew ! Before i posted the question, i did go through that link .If you please refer to the link , point 3 says

"After max-age timer expires,the bpdu stored on "s" for port p ages out. The port goes immediately to listening and "s" starts to send its better bpdu to "b" .

I read somewhere switches send bpdu every two seconds so why "s" waits for 20 seconds before sending its better bpdu to " b" ?

I really appreciate Andrew if you could elaborate on it.

thanks a lot !

OK - here goes.....

When a root switch is elected, the rest of the switches need to determine their Root Port. The root switch creates and sends a hello every 2 seconds - this is the default setting.

If something happens to the Root switch, the rest of the switches will not hear any Hello messages, and after the Maxage timer expires(default 10 times Hello, or 20 seconds) the switches elect a new root using BPDU's?

HTH.

sarahr202 Thu, 07/10/2008 - 05:17

thanks for your reply! The point is switch"s" is receiving bpdu from root, but it has to wait 20 seconds before it transmits the bpdu on its blocked port connected to switch "p" whose root

port has gone down and switch "p" is sending inferior bpdu claiming to be root, to switch "s"

As you said every switch sends bpdu every two seconds so this is not the case here ?

thanks a lot!

The point is the Maxagetimer - regardles, it will wait for begin the root election process.

If an inferior bpdu is recevied from a switch "claming" to be the root switch, the switch that is actaully still connected to the root, will ignore this, and re-send the root's bpdu.

On the first link I sent to you, the first example:-

1.

If link L1 goes down, switch B immediately detects the failure and assumes it is the root. It starts to send BPDUs to S and claims to be the new root.

2.

When S receives this new BPDU from B, it realizes it is inferior to the one it had stored for port P and ignores it.

3.

After max_age timer expires (20 seconds by default), the BPDU stored on S for port P ages out. The port goes immediately to listening and S starts to send its better BPDU to B.

4.

As soon as B receives the BPDU from S, it stops sending its BPDU.

5.

Port P moves to the forwarding state through listening and learning states. This takes twice the fw_delay value, an additional 30 seconds. Full connectivity is then restored.

HTH.

bvsnarayana03 Thu, 07/10/2008 - 07:29

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.

sarahr202 Thu, 07/10/2008 - 13:02

thanks for your reply. My point is switches send bpdu every two seconds.Based on your reply, i understand s2 is not sending bpdu every two seconds untill max-age timer expires. Am i correct to assume switch normally send bpdu every two seconds but when it recives inferior bpdu, it stops sending bpdu and waits untill max -age timre expires. once the max-age timer expires, switch start sending bpdu every two second ?

thanks a lot !

sarahr202 Thu, 07/10/2008 - 13:42

hi every body !

please disregard my last post. I forgot that port in block state can only receive bpdu not transmit until max-age timer expires. Now it makes complete sense to me.

thanks a lot and have a wonderful day !