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Is this answer from the book wrong ? Designated ports

Hi, can you confirm this for me ?

"Diagram is attached. Which port will be elected as non-designated port ?

a)1/1

b)1/2

c)2/1

d)2/2

e)3/1

f)3/2

"

According to the book, answer is b).

In my view, I thought that there are two non-designated ports

on this; b) and f)

That happens because the 'bridgeID:1' switch becomes the root

because it has the lowest bridgeID. Ports 2/1 and Port 2/2 on

bridgeID:1 become 'designated' ports.

BridgeID:3 port 1/1 is a Root Port.

BridgeID:2 port 3/1 is a Root port.

BridgeID:3 port 1/2 is a NON DESIGNATED PORT - I Agree.

How about this one:

BridgeID:2 port 3/2. In my view that should be NON-DESIGNATED port as

well, isn't it ?

Can you please confirm what I am missing here or whether the book

is wrong and there really two answers for this ?

3 ACCEPTED SOLUTIONS

Accepted Solutions

Re: Is this answer from the book wrong ? Designated ports

Hi Friend,

THe answer from the book is correct.

When the loop has to be broken the criteria used by STP for selecting the designated port is

1) root switch

2) Shortest path to the root switch

3) sending bridge ID

4) Sending port ID

Since in the above case, the first 2 parameters are equal, the bridgeID is considered and hence port 3/2 on B2 is put into forwarding and the port 1/2 on B3 is put into blocking which eleminated the loop. If there is a link failure between B3 and B1 this blocked port will be put into forwarding again and the connectivity will be restored.

THe understanding here is STP will not block ports on both the side of the link to remove a loop. The port will be blocked on one side of a segment and will be kept into forwarding on the other side of the segment.

HTH, rate if it does

Narayan

Cisco Employee

Re: Is this answer from the book wrong ? Designated ports

Hi Friend,

First to confirm that answer is correct and in addition to Narayan post after the choice of the designated port for a network segment is done, any other ports that connect to that segment become nondesignated ports.

These ports block network traffic from that path so that the traffic can only access the segment through the designated port.

So as Narayan explain the reason WHY 3/2 will be fwding 1/2 will be in blocking state to avoid loop and block network traffic from that path so that the traffic can only access the segment through the designated port which is 3/2.

HTH

Ankur

*Pls rate all helpfull post

Re: Is this answer from the book wrong ? Designated ports

Hi Friend,

No the port 3/2 will not be a NON-Designated port. It will be a designated port for the Segment connected to the switches with bridgeID 2 and 3. A non-deignated port is a port which goes to blocking state and in this scenario 3/2 is a forwrading designated port.

The Answer is B

Please see the link below for the clear understanding of your doubt.

http://www.cisco.com/warp/public/473/spanning_tree1.swf

HTH, Please rate if it does.

4 REPLIES

Re: Is this answer from the book wrong ? Designated ports

Hi Friend,

THe answer from the book is correct.

When the loop has to be broken the criteria used by STP for selecting the designated port is

1) root switch

2) Shortest path to the root switch

3) sending bridge ID

4) Sending port ID

Since in the above case, the first 2 parameters are equal, the bridgeID is considered and hence port 3/2 on B2 is put into forwarding and the port 1/2 on B3 is put into blocking which eleminated the loop. If there is a link failure between B3 and B1 this blocked port will be put into forwarding again and the connectivity will be restored.

THe understanding here is STP will not block ports on both the side of the link to remove a loop. The port will be blocked on one side of a segment and will be kept into forwarding on the other side of the segment.

HTH, rate if it does

Narayan

Cisco Employee

Re: Is this answer from the book wrong ? Designated ports

Hi Friend,

First to confirm that answer is correct and in addition to Narayan post after the choice of the designated port for a network segment is done, any other ports that connect to that segment become nondesignated ports.

These ports block network traffic from that path so that the traffic can only access the segment through the designated port.

So as Narayan explain the reason WHY 3/2 will be fwding 1/2 will be in blocking state to avoid loop and block network traffic from that path so that the traffic can only access the segment through the designated port which is 3/2.

HTH

Ankur

*Pls rate all helpfull post

Re: Is this answer from the book wrong ? Designated ports

Hi Friend,

No the port 3/2 will not be a NON-Designated port. It will be a designated port for the Segment connected to the switches with bridgeID 2 and 3. A non-deignated port is a port which goes to blocking state and in this scenario 3/2 is a forwrading designated port.

The Answer is B

Please see the link below for the clear understanding of your doubt.

http://www.cisco.com/warp/public/473/spanning_tree1.swf

HTH, Please rate if it does.

New Member

Re: Is this answer from the book wrong ? Designated ports

What a nice presentation. Thanks for sharing this ! I wish they had more like this one.

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