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New Member

Spanning-Tree conundrum

I am reading my CCNP Switch book and it describes the following STP setup between four switches:

It then goes on to describe what happens when the link from A1 to D1 goes down.

My question is... how does A2 detect the indirect link failure?

It has a good link to the root so it will not give up its latest BPDU.

Will A1 start to advertise itself as the root? Even if it does A2 will not accept it.

My suspicion is that D2, having lost its connection to the Root Bridge will wait the max age time of 20 seconds. Then it will recieve a superior BPDU from switch A2. It would then transiton the link to the forwarding state. But the D2 link to A2 is designated. Not blocking. So I'm unsure how that would work. Plus the text says that A2 and not D2 detects the failure.

I suppose what I'm asking is... how does a switch (in this case A2) detect and an indirect link failure if its own link to the root bridge is not disconnected?

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New Member

Spanning-Tree conundrum

I have just realised something....

On the link between A2 and D2 .... the A2 end would surely be the designated port.

It of course has a lower path cost to the root. If D2 had a better cost A2 would go through D2 to get to the root but it doesn't.

My thinking is that the text book should have said that D2 will detect the failure and bring its state from Block to Forwarding over 50 seconds.

Does this make sense? The text clearly states that it is A2 that brings the link back up. Maybe I am misunderstanding something here...

Spanning-Tree conundrum

Since D1 is the root switch, the port going from A1 to D1 will be the root port. This means the port going from A1 to D2 will be in the blocking state since it has a redundant path to the root switch through (A1 -> D2 -> A2 -> D1). If the failure happend directly on A1, it will move the port going to D2 into a forwarding state after 30 seconds. While its transition from blocking to forwarding A1 will declare itself the root switch since it stops hearing bpdus telling it otherwise. Once it finally transitions to the forwarding state (after 30 seconds Listening+Learning), it will send bpdus towards D2 declaring itself as root. Now, D2, will say HEY!!, wait a minute, this is an inferior bpdu and ignore the bpdus. After 20 seconds (max_age), The bpdu stored on D2 will age out, (remember bpdus are stored per port so to speak), it will then send out a bpdu with D1 as the root switch (remember D2 still has a path to the root switch so it's still receiving better bpdus), A1 will then accept this BPDU and the port from A1 to D2 will transition to the root port.

Now, if the failure happend on the root switch D1, A1 will not know about this for 20 seconds (max_age) and then start the process as described earlier.

Hope this helped.

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