BCMSN - STP question

Unanswered Question
Jul 2nd, 2008

Now studing for the BCMSN with the CiscoPress BCMSN Vol. 4 pg. 187 Figure8-2.

My question refers to a scenario where PC1 is connected to 2 switches for redundancy and forwarding a frame to the PC4 on the other side.

I understand the concept of a switching loop ...but this has me hung up.

It says "lets assume that both switches know the mac address both of the PCs in question. It then states that "both switches will receive the frame on their incoming ports". Then it will forward the frames on to PC4 and PC4 will receive both frames. It states "this is not ideal, but not disasterous either".

Question 1 : How/Where did the frame get duplicated? (the frame is received on both incoming switch interfaces)

Am I missing somthing here?

Any help is appreciated.

I have this problem too.
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Giuseppe Larosa Wed, 07/02/2008 - 12:36

Hello Sean,

this example is provided to show the need for the STP protocol.

Here the two switches are two ideal transparent bridges and they are unaware of the presence of each other.

(this was written clearly on the first edition of the book I think )

So the duplication is provided by the fact that both switches receive the frame and both send out on the other interface.

Then both switches update their CAM table because each receives the frame generated by the other one.

In a real world experiment this doesn't happen because the two switches know they are both connected to the two LAN segments (by STP) and only one is in forwarding state for any segment.

This is one of the purposes of STP to build a loop free topology.

So a switch is not allowed to modify a user frame but can send and receive signalling frames as needed (from STP, CDP, VTP, DTP, PAGP ...)

hope to help


asgerbert Wed, 07/02/2008 - 12:49

Thanks for the explanation. I knew the frames were not duplicated... but the books leads you to believe that somewhere they were.

Thanks again.

asgerbert Wed, 07/02/2008 - 13:00

Just to clear this up for me...

Am I correct in saying that the same original frame that was transmitted, IS NOT received by both switches at the same time?

Only one frame was transmitted...How can both switches receive the frame when only one was xmitted?

That is now my understanding and source of confusion.

ralphcarter Wed, 07/02/2008 - 12:38

When the switch doesn't know where the destination address is, it floods a broadcast frame to all switchports in the vlan. Each switch hears the broadcast reply and records the source mac/port.

In this case there is loop in the topology and the frame looks like its duplicated.

asgerbert Wed, 07/02/2008 - 12:45

So... In essencse it is NOT received on both switch interfaces. It is a broadcast that initiates the other switch to add the MAC to the CAM table. Thus example why you need STP.

The way the book states it the PC sends the frame then it "is received on both interfaces of the switches". This leads you to believe that the frame was "duplicated" somewhere because the next paragraph says "PC4 receives both frames".

Very confusing way to put it.

Thanks for clearing this up.

Giuseppe Larosa Sat, 07/05/2008 - 04:20

Hello Sean,

the two switches can receive the frame both: imagine that the pc is connected to a hub (the lan segment) and then the hub is connected to two ports one on switch1 and the second on switch2.

Then the two switches send out on the other network segment where again PC4 is connected to a hub anf then the hub is connected to two ports one of each switch.

The two switches don't speak STP and don't know of each other.

If you add this to the example it should like more reasonable.

By the way, Unkwown unicast MAC addresses are sent out all ports in the same vlan without any change to the frame (with the exception of the one where the frame is received) this is also known as unknown unicast flooding.

It is treated like a broadcast but it isn't a broadcast.

As soon as someone starts to send out frames with a MAC source equal to the unknown one, the association is built and all other frames with this destination are not flooded anymore but sent to the port where the station is.

Hope to help



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