Designated switch

[rezaalikhani]

Hi all,

I am preparing for the CCNA exam and currently studying STP. I have a problem in understanding Designated switch and designated port concept. I have read several documents and books, but unfortunately none of them helped me to understand this feature.

Can anyone describe me this feature?

Thanks

Reza

[rolf.fischer_2 · Accepted solution]

Hi,

do you know the STP flash-animation at http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml?

It's realy helpful for understanding STP.

Another great resource is http://iws.ccccd.edu/sbutler/courses.html.

Good luck for your exam!

Regards

Rolf

[Jon Marshall · Accepted solution]

Hi Reza

Root ports on switches lead to the root bridge. Designated ports lead away from the root bridge. eg

You have 3 switches A B & C

A is the root bridge.

B has a root port of fa0/1 which is the connection to Switch A.

C has a root port of fa0/1 which is the connection to switch A.

B & C are connected to each other using ports fa0/2 on both ends.

So we have a loop in the network ie.

traffic sent from A to B will be forwarded onto C and forwarded onto A etc...

Now, one of the FA0/2 ports on switch B or C will be become the designated port for that network segment and be responsible for sending and receiving traffic on that segment. The other fa0/2 port will be placed in a blocking state.

So let assume that after the BPDU's have been transferred the fa0/2 interface on Switch B is chosen as the designated port.

Switch B is now the designated bridge for that lan segment as it has the designated port.

So fa0/2 on switch C is now put in blocking mode and you have effectively removed the layer 2 loop.

Hope this makes sense

Good luck with exam

Jon