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?
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
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!